Textured surfaces for display applications

ABSTRACT

A substrate with a textured surface is disclosed. The substrate may be, for example, a light emitter comprising a light guide, for example a backlight element for use in a display device, wherein a surface of the light guide, for example a glass substrate, is configured to have a textured surface with a predetermined RMS roughness and a predetermined correlation length of the texture. A plurality of light scatter suppressing features can be provided on the textured surface. Textured surfaces disclosed herein may be effective to reduce electrostatic charging of the substrate surface. Methods of producing the textured surface are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/792,167 filed on Oct. 24, 2017 which is a divisional of U.S. patentapplication Ser. No. 15/105,025 filed Jun. 16, 2016 which claims thebenefit of priority under 35 U.S.C. § 371 of International PatentApplication Serial No. PCT/US14/70771, filed Dec. 17, 2014, which claimsthe benefit of priority under 35 U.S.C. § 119 of U.S. ProvisionalApplication Ser. No. 61/918,276 filed on Dec. 19, 2013, the content ofwhich is relied upon and incorporated herein by reference in theirentirety.

BACKGROUND Field

The present invention relates generally to textured surfaces for displayapplications. For example, a textured surface as disclosed herein may beused in a light emitter (e.g., a backlight for certain display devices),the light emitter comprising a substrate with a textured surface on oneor both major surfaces of the substrate that can operate as a lightguide, and processes for making the light emitter. The light guide mayinclude a pattern of discrete dots of material deposited thereon tocontrol the light output of the light emitter. Textured surfacesdisclosed herein may also be used as projections surfaces for viewingimages from a projecting image source. In certain embodimentsmicro-replication (nano-replication) techniques may be used toefficiently and economically produce the textured surface. Texturedsurfaces disclosed herein may even be used to mitigate electrostaticcharging for display glass substrates during certain manufacturingprocesses.

Technical Background

Conventional components used to produce diffused light have includeddiffusive structures, including polymer light guides and diffusive filmswhich have been employed in a number of applications in the displayindustry. These applications include bezel-free television systems,liquid crystal displays (LCDs), electrophoretic displays (EPD), organiclight emitting diode displays (OLEDs), plasma display panels (PDPs),micro-electromechanical structures (MEMS) displays, electronic reader(e-reader) devices, and others.

The desire for thinner, lighter and more energy efficient displays haveled to the development of so-called transparent displays. Long a stapleof science fiction, transparent displays are now being commerciallyimplemented in several variations, including vending machine doors,freezer doors, retail advertising, augmented reality screens, heads-updisplays in the automotive industry, smart windows for offices, portableconsumer electronics, and security monitoring.

Unfortunately, transparent displays are susceptible to several poorperformance characteristics. In actuality, currently available displaysonly partially transmit and reflect light, thus the contrast ratio ofthe display is greatly limited. Commercially available transparentdisplays typically offer only about 15% transmission, and performance iseven lower in reflection mode.

For many practical applications, a transparent display requires thesupport of backplane illumination (via a transparent back lightelement). To maintain transparency, the back light needs to be fullytransparent in an OFF-state and fully illuminated in an ON-state. Backlights having a frosted appearance are generally unacceptable.Additionally, the use of a transparent back light necessarily eliminatesthe use of a conventional reflective medium. Existing technology forproviding backplane illumination are not satisfactorily meeting certaincost and performance requirements of the marketplace for transparentdisplays.

Other components of display devices, particularly the substrates formingthe display panel itself, may present problems during manufacture of thethin film devices formed on at least one of the substrates. For example,triboelectrification can impede the ability to remove the substrate fromsurfaces in contact with the substrate, which can lead to breakage ofthe substrate and particle generation in some cases. This can beparticularly troublesome for substrates, such as glass substrates,produced via processes that result in extremely smooth surfaces on thesubstrate.

SUMMARY

Conventional transparent display systems, such as transparent andsemi-transparent LCD televisions, are commercially available for digitalsignage and advertising applications. These display systems aresemi-transparent in the OFF-state (i.e., when no image is beingcommanded by the associated electronics driving the LCD elements). Tomaintain the semi-transparent characteristics, these conventionaldisplay systems do not employ an opaque optical light emitter to producelight. Instead, these display systems use background ambient light toilluminate the LCDs in the ON-state (i.e., when the associatedelectronics is commanding an image), a so-called “white box” approach.In the white box approach, a large “box”, such as a food vendingmachine, includes one or more reflective interior surfaces, for examplesurfaces coated with white paint, and the box is brightly lighted fromwithin. The window to the box, for example a door, comprises atransparent display panel, wherein the brightly lighted box serves as alight source for the display window. Thus, one can see through thedisplay and view objects (such as merchandise, etc.) behind the displaypanel. Concurrently, the viewer can also receive visual information oncertain portions or the entirety of the display panel, which in acommercial application might be related to the merchandise behind thedisplay panel.

As discussed above, a significant issue with current “transparent”display systems is that they are not particularly transparent. In fact,measurements have shown they exhibit only about a 15% transmissionratio. Thus, a relatively high level of ambient light may be needed inproximity to the display panel, which might not always be feasible oreven desirable for any number of reasons. The result is an array ofproblems related to uniformity of image quality across the display, suchas non-uniformity in color, contrast ratio, etc.

Thus, in accordance with one or more embodiments presented herein, alight emitter is described. As used herein, the term light emitterrefers generically to a device configured to provide illumination. Thelight emitter may, for example, include a back light element for use ina display device, or the light emitter may be configured to providegeneral illumination such as illumination for a room or vehicle. Thelight emitter includes a light guide into which light can be coupled byone or more slight sources and through which the coupled lightpropagates. The light guide is generally a substrate comprising opposingmajor surfaces. As used herein, the term substrate refers generally to aplate-like substrate and which in some embodiments is suitable for useas a light guide. The substrate may be planar, or the substrate may, insome examples, have a wedge-like shape. Nevertheless, the substrategenerally has two opposing major surfaces bounded by edge surfaces. Themajor surfaces may be planar and parallel, or the major surfaces may beplanar and non-parallel. In some embodiments the substrate may becurved, for example as used in a curved display device. For lightemitter applications at least one of the opposing major surfacesincludes a surface texture configured to scatter at least a portion ofthe light propagating within the light guide. The surface texture isspecifically configured to render the light guide visually transparent,without appreciable haze, thereby making the light guide particularlyuseful in the construction of a back light element for use in atransparent display device. The configuration of the surface texturealso provides excellent viewing angle performance.

The light emitter, for example a back light element comprising a lightguide, may be positioned behind a transparent display panel relative toa viewer of the display panel. Light may be coupled into the light guidealong one or more edge surfaces of the substrate, and/or along one ormore borders thereof, wherein the borders represent portions of themajor surfaces proximate the edge surfaces. The light propagates in awaveguide fashion within the light guide, for example by total internalreflection, and is incident on the light scattering portion of the atleast one major surface. Thus, light propagating through the light guideand which light may be incident on a textured surface of the substratemay be scattered out of the light guide to illuminate the display panelof the display device, such as an LCD display panel. The light emitterdescribed herein can function as an improved light source for the LCDelements of a transparent display system over currently availablealternatives by eliminating the need for complex scattering and turningfilms, which decrease the light output from conventional back lightelements. In addition, the surface texture produces a level of haze lowenough to achieve a high level of back light element transparency whenportions of the display system are in the OFF-state. In examples, thehaze may be equal to or less than 6%, equal to or less than 4%, equal toor less than 2% or equal to or less than 1%, it being understood thathaze is bounded at its minimum by 0%. In most cases, a haze equal to orless than 1% is desired. As used herein, the term transparent, orvariations thereof, is intended to convey visual transparency (forexample over a wavelength range from about 400 nm to about 700 nm),wherein an object placed behind the transparent element can bedistinctly seen by an observer from the opposite side of the transparentelement, the transparent element being disposed on the sight linebetween the observer and the object.

In use as a back light element, the light emitter can provide additionallighting output over conventional back light elements to increase thebrightness, functionality and the viewing angle of a transparent displaysystem. In other uses, the scattering structure of the light guide andits use in a light emitter can be employed in a variety of applicationsas a source of light, including but not limited to architecturallighting, automotive lighting, decorative lighting and the like.Accordingly, the light emitter in various aspects described herein isnot limited to a display back light element, but may be used in avariety of different applications where a source of light may be needed,the foregoing list naming but a few. Moreover, display applications mayinclude devices extending beyond televisions, computer displays, laptop,tablet and phone displays and can include, for example, sky lights,virtual reality goggles or other wearable display devices.

In the context of a display device, the addition of a visuallytransparent back light element can eliminate the need for a reflectivemedium in the transparent display, which eliminates a major disadvantageof reflective-type displays). The transparent display may therefore worksolely in transmission, eliminating image distortion from glare andreflection. Another advantage to eliminating the reflective medium is inproviding an improved color balance. By working in transmission-onlymode, the transparent display device can have an improved imagerendering quality.

Still further, the light emitters described herein can provideimprovements in color fidelity by enabling the tuning of light coupledfrom the light source into the light guide. When such tuning includesselecting colors to mix with the natural color shade of the transparentdisplay device, the color fidelity of the display device is improved.Additionally, in one or more further embodiments, the light coupled intothe light guide may be adaptive (by tuning the light sources, e.g.,controlling LEDs of different colors) to enhance scene colors based oninstantaneous color content.

The primary advantage of an etching approach according to the presentdisclosure for making textured surfaces, such as textured surfaces fordisplay applications, and particularly when the substrate is glass, isthat it can be accomplished with only wet solutions using chemicalscommonly available in industrial quantities. The use of commonlyavailable chemicals can significantly lower the cost of the processcompared with other techniques, such as abrasive etch, etchingparticles, and/or aqueous etching cream processes. The solutionsrequired for the etching techniques disclosed herein may also notinclude a common but very hazardous hydrofluoric acid component, whichresults in a much safer etch mixture for production both in terms ofworker safety and environmental quality. The process can produce anarrow distribution of lateral feature sizes when compared with someother processes.

In architectural and aesthetic applications, light emitters as describedherein may be employed for both indoor and outdoor lighting, bothfunctional and decorative. For example, the light emitter may be used asa skylight, where the textured glass would be relatively unaffected byweather conditions. The advantages of using a substantially transparentlight emitter as a skylight include high resistance to discolorationfrom weathering, the ability to provide artificial light (not simplychannel sunlight) in low lighting conditions, such as at night or oncloudy days to provide additional lighting. The transparency of a lightemitter employed as a skylight can allow viewing the overhead sky duringdaylight hours, yet provides illumination to the underlying space duringlow light or evening hours. Moreover, an advantage of light emittersdescribed in certain examples herein is the ability to provide uniformlight output across a surface thereof, if so desired, by the applicationof light scatter suppressing features on the textured surface of thelight guide. As the name implies, such light scattering featuresfunction to suppress or completely eliminate scattering of incidentlight at the feature location. In some embodiments, such as decorativeapplications, light output from the light emitter may be intentionallyconfigured to be non-uniform, either by eliminating the light scattersuppressing features, or by the intentional placement of a pattern ofthe light scatter suppressing features so as to produce a desiredeffect. The range of such designed patterns is limited only by thedesigner's imagination.

In yet another application, certain transparent substrates can beutilized in a projection mode, wherein a projection system projects animage onto a visually transparent substrate such that the transparentsubstrate functions as a projection screen. In such applications theprojected image is readily viewable on the transparent substrate, andyet the substrate is simultaneously substantially transparent so thatthe background behind the screen is clearly visible. Ordinarily, such animage projected onto a transparent substrate, for example a glass platenot configured to scatter light, will not be readily visible at thetransparent substrate. Instead, light rays from the projection systemwill be substantially transmitted through the substrate making itdifficult if not impossible to view the projected image on thesubstrate. Textured substrates as described herein, however, are capableof providing sufficient scattering of the incident light to make theimage visible, while maintaining the substrate essentially transparentto the viewer without discernible haze. Touch capability can be added tothe projection screen by adding touch-functional layers to theprojection screen in a manner as is known in the art. Such touchfunctionality can be accomplished by using transparent oxide materialsthat do not significantly affect the transparency of the projectionscreen.

In still other applications, substrates utilizing textures disclosedherein can be positioned in front of but spaced apart from an imagedevice, for example a large screen television device or a computermonitor, as an overlay. Again, touch capability can be incorporated withthe substrate which can be electronically coordinated with the displayedimage, thereby adding touch capability to the image device that was notpreviously present. The substrate can be entirely structurallydisconnected from the image device, although electrical communicationmay be required. Accordingly, a controller can be used to relate thetouch (contact) location on the overlay panel with a particular locationon the display panel image being displayed immediately behind that touchlocation, and then initiate a predetermined action.

Accordingly, the light emitter for these and other uses, and moreparticularly the light guide comprising the light emitter, can, inaddition to being transparent (e.g. exhibit minimal haze), be configuredto be suitably rigid and may, according to some examples, have a bulkmodulus in a range equal to or greater than 10 gigaPascals (GPa). Forexample, the bulk modulus can range from about 10 GPa to about 445 GPa.For example, in a range from about 30 GPa to about 250 GPa, or in arange from about 30 GPa to about 100 GPa. Glass, for example, has a bulkmodulus in a range from about 35 to about 55 GPa. In contrast, sapphirehas a bulk modulus of about 250 GPa.

To ensure suitable transmittance within the interior of the light guide,the light guide may have an optical loss equal to or less than 26 dB permeter in a wavelength range from about 400 nanometers to about 700nanometers. In some examples the optical loss can be equal to or lessthan 17 dB per meter, whereas in certain other examples the optical losscan be equal to or less than 10 dB/meter. Suitable materials includewithout limitation glass materials including silica-based glasses,crystalline materials such as sapphire or diamond and diamond-likematerials, or other materials, including calcium fluoride, as thequality of light emitted from the light guide is more a function of thesurface characteristics rather than the bulk material.

A scatter ratio of the light guide, defined as the diffuse transmittancedivided by the total transmittance, may be equal to or greater than 0.5.Moreover, for applications needing uniform illumination, a brightnessvariation of the light guide across an emitting surface of the lightguide can be equal to or less than 20%. A viewing angle goodnessparameter (VAG), described herein below, may be equal to or greater than1.0.

Still further, particular processes for producing glass light guidesdescribed herein are compatible with both pre and post-procession-exchange treatments. The use of chemically strengthened glassthrough ion exchange may be used to provide protection for otherdelicate structures, such as the LCD panel in a display device.

Compared to other techniques, illumination using light emittersdescribed herein can be very uniform, light transmission is high, and,in the case of glass, the size of the substrate that can be prepared isnot particularly limited.

Accordingly, disclosed herein is a substrate comprising at least onetextured major surface comprising an RMS roughness R_(q) where 5nanometers≤R_(q)≤75 nanometers, for example in a range from about 5nanometers to about 40 nanometers. The texture of the textured majorsurface may also include a correlation length T where 0 nanometers<T≤150nanometers, for example in a range from greater than zero to equal to orless than 100 nanometers. The substrate may further comprise a hazevalue equal to or less than 6.0% and a transmittance normal to the atleast one major surface greater than 90% over a wavelength range from400 nm to 700 nm. The substrate may comprise a glass layer. Thesubstrate may comprise a polymer layer. For example, the substrate maycomprise a glass-polymer laminate including a polymer layer disposed onthe glass layer.

In some embodiments the textured major surface is a surface of a polymerlayer.

In certain examples the substrate can comprise a first substrate and mayfurther comprise a second substrate joined to the first substrate toform a substrate assembly with the textured surface positioned within aninterior of the substrate assembly. For example, the textured surfacecan be a surface of a polymer material disposed on the first substrate,and a second substrate is positioned such that the polymer layerincluding the textured surface is sandwiched between the first andsecond substrates. Both substrates may include a glass layer. Forexample, in some embodiments the first substrate may be a glass plateand the second substrate may be a glass plate. The polymer layer may be,for example, a pre-textured film produced in a replication process, forexample a micro- or nano-replication process, and positioned between thefirst and second glass plate.

The substrate may be a chemically strengthened substrate, for example anion-exchanged glass substrate. In some embodiments the substrate may bea laminated substrate comprising a first layer including a firstcoefficient of thermal expansion and a second layer comprising a secondcoefficient of thermal expansion different from the first coefficient ofthermal expansion. For example, the laminate may be a glass laminatewherein two layers of glass having two different compositions and twodifferent coefficients of thermal expansion may be fused together toproduce the laminate. The glass laminate can be produced, for example,via a fusion process wherein multiple streams of molten glass ofdifferent compositions are flowed over the surfaces of forming bodiesand joined to produce a single glass stream that cools and can be cutinto individual glass sheets. The differing coefficients of thermalexpansion can produce a compressive stress in the surfaces of the glasssheets.

The polymer layer according to some embodiments may comprise a pluralityof discrete light scatter suppressing features, the discrete lightscatter suppressing features comprising an index of refractionsubstantially equal to an index of refraction of the glass layer. Forexample, the index of refraction of the light scatter suppressingfeatures can be within ±10% of the index of refraction of the substrate.A spatial density of the plurality of light scatter suppressing featurescan be arranged to vary as a function of distance from an edge of thesubstrate to produce a predetermined light output as a function ofposition on a surface of the substrate. For example, a spatial densityof the plurality of light scatter suppressing features can be arrangedto produce a substantially uniform light output for use as a back lightfor a display device.

In other embodiments the polymer layer may be a continuous layer, suchas a polymer film applied to a glass layer, e.g. the first glasssubstrate. The second substrate may be a chemically strengthenedsubstrate. For example, in embodiments comprising two glass substrates,both glass substrates can be chemically strengthened substrates.Alternatively, the second substrate is a laminated substrate comprisinga first layer including a first coefficient of thermal expansion and asecond layer comprising a second coefficient of thermal expansiondifferent from the first coefficient of thermal expansion. Both thefirst and second substrates can be laminated substrates comprising aplurality of glass compositions with different coefficients of thermalexpansion.

Preferably, the substrate comprises a bulk modulus M, where 10gigaPascals M 450 gigaPascals. The substrate preferably comprises ascatter ratio defined as a diffuse transmittance divided by a totaltransmittance equal to or greater than 0.5. The substrate preferablycomprises an optical attenuation equal to or less than 26 dB/meter in awavelength range from about 400 nm to about 700 nm.

When used as a light emitter, for example a light guide in a back lightelement, the substrate may further comprise a light source, or aplurality of light sources, positioned adjacent an edge surface of thesubstrate.

In some embodiments the substrate may further comprise a frame disposedabout a perimeter of the substrate. For example, the substrate may beconfigured as a projection screen or a display overlay.

In another embodiment a light emitter is disclosed comprising a lightguiding substrate comprising a first edge and a second edge opposite thefirst edge, the substrate further comprising at least one texturedsurface with an RMS roughness in a range from about 5 nanometers toabout 75 nanometers, for example in a range from about 5 nanometers toabout 40 nanometers. A correlation length of the texture may be in arange from greater than zero nanometers to about 150 nanometers, forexample in a range from about 5 nanometers to about 100 nanometers. Thesubstrate may, for example, be a glass substrate, and more particularlythe glass substrate may be a silica-based glass substrate. The substratemay include a plurality of discrete deposited features on the texturedsurface that function as light scatter suppressing features. A spatialdensity of the deposited features may vary as a function of distance ina direction from the first edge to the second edge. For example, thespatial density of the deposited features may decrease as a function ofdistance in a direction from the first edge to the second edge. In oneexample, the spatial density of the deposited features may vary fromabout 95% to about 5% in a direction from the first edge to the secondedge, the spatial density determined as a percent coverage of a unitarea. The spatial density of the deposited features may be substantiallyconstant along a line parallel with the first edge. The depositedfeatures may be locally randomly distributed.

Preferably, the deposited features are transparent. The depositedfeatures may comprise, for example, a polymer resin. An index ofrefraction of the deposited features may be close to or match arefractive index of the substrate. For example, an index of refractionof the deposited features may be within 10% of the index of refractionof the substrate. Typically, a refractive index of the substrate is in arange from about 25×10⁻⁷/° C. to about 300×10⁻⁷/° C. over a temperaturerange from 25° C. to 300° C. The light emitter may be transparent suchthat in an OFF state the light emitter can be viewed through, and in theON state the light emitter emits light.

As will become clear from the following description, the light emitterwhen configured as a back light element may also function as an opaquelight emitter by employing a reflective panel at the back side of thelight guide farthest from the display panel. Accordingly, the lightemitter disclosed herein may be used where transparency is not requiredand a greater light output is needed.

The substrate may include a surface porosity characterized by a bimodalparameter (BP) value in a range from about 0.16 to about 0.22.

In another aspect, a display device is disclosed comprising: a displaypanel, a light emitter, for example a back light element positionedadjacent to the display panel, the back light element comprising a lightguide including a substrate with a first edge and a second edge oppositethe first edge. The substrate further comprises at least one texturedsurface with an RMS roughness in a range from about 5 nanometers toabout 75 nanometers, for example in a range from about 5 nanometers toabout 40 nanometers. A correlation length of the surface texture may bein a range from greater than zero nanometers to about 150 nanometers,for example in a range from 5 nanometers to about 100 nanometers Thetextured surface may include a plurality of discrete deposited featuresdeposited thereon to control light scattering from the light guide. Thedeposited features serve as light scatter suppressing features. Forexample, the deposited features may be used to selectively control thescattering of light from the textured surface of the substrate.Accordingly, the deposited features may be configured to produce auniform illumination from the light emitter. Thus, a spatial density ofthe deposited features can decrease as a function of distance from thefirst edge to the second edge. In other examples the deposited featuresmay be arranged to produce an intentional pattern of illumination, forexample a non-uniform pattern such as for decorative lighting. Indisplay applications the textured surface of the light guide may bepositioned to face away from the display panel. In other examples thetextured surface can be arranged to face toward the display panel. Instill other examples, both major surfaces of the light guide may betextured. For use in applications where transparency is not a benefit,the light emitter may be opaque and incorporate, for example areflective element positioned adjacent a major surface of the lightguide to increase light output of the light emitter.

To ensure proper control of light scattering by the light guide, andtherefore light extraction, the deposited features may comprise an indexof refraction close to or matched with the index of refraction of thesubstrate. For example, the index of refraction of the depositedfeatures may be within 10% of the index of refraction of the underlyinglight guide substrate and be considered refractive index-matched. Thedeposited features may, for example, be a polymer resin. In otherexamples the deposited features may include transparent metal oxidelayers such as a transparent conductive oxide (TCO) used in themanufacture of semiconductor devices.

The spatial density of the deposited features can vary from about 95% toabout 5% in a direction from the first edge to the second edge, forexample in a range from about 75% to about 25%, the spatial densitybeing determined as a percent coverage of a unit area. Thus, the spatialdensity may be a gradient from one edge in a direction toward anotheredge, or even from one edge to a middle of the glass substrate betweentwo parallel edges. As used herein, an edge may be distinguished from anedge surface in that an edge represents the boundary between an edgesurface and a major surface of the substrate. For example, although insome embodiments illustrated in the accompanying drawings the lightguide is shown with light coupled from the light source(s) through asingle edge surface of the substrate, in other embodiments not shown,light may be coupled from two or more edge surfaces of the substrate.Suitable light sources include light emitting diodes (LEDs). In someexamples, light may be coupled from a border area of a major surfaceproximate an edge (perimeter) of the major surface. Accordingly, thepattern of the deposited features, and therefore the spatial densitythereof, may be adjusted to obtain the desired light output uniformity.However, on a local scale, the deposited features may be randomlydistributed.

In still another embodiment a method of forming a light guide isdescribed comprising treating a surface of a glass substrate with anetchant comprising acetic acid, for example glacial acetic acid, in anamount from about 92% to about 98% by weight, ammonium fluoride in anamount from about 0.5% to about 5.5% by weight, and less than 6% byweight water. While not wishing to be bound by any particular theory, itis believed that for some embodiments the lower the water content, thelower the ammonium fluoride dissociation rate, which leads to a reducedrelease of HF. A reduced concentration of HF can also lead to areduction in the removal of glass from the glass substrate, which inturn results in shallower pits in the etched surface thereof, which isneeded for low haze (e.g. greater transparency). Additionally, reducedwater content can provide for a reduction in the solubility of theprecipitated crystal mask in the etchant. This means the mask stays onthe glass substrate surface longer. When the nucleated crystals stayundissolved on the glass surface, the etch rate for the etchant is lowerand the crystal lateral dimension remains unchanged, thus leading to thedesired low correlation length.

The method may further comprise depositing a plurality of dots(deposited features) of a transparent material on the textured surfaceof the glass substrate. The deposited features serve as light scattersuppressing features. For example, the deposited features may be used toselectively control the scattering of light from the textured surface ofthe substrate. The dots comprising the deposited material may have anindex of refraction that is close to or matches a refractive index ofthe glass substrate, for example within ±10%. A spatial density of thedeposited features preferably decreases as a function of distance from afirst edge of the glass substrate to a second edge of the glasssubstrate parallel with the first edge to produce a graduated spatialdensity. The deposited dots can, for example, comprise a polymer resin.The graduated spatial density may comprise a linear graduation or anon-linear graduation.

The spatial density of the deposited features can vary from about 95% toabout 5% in a direction from a first edge of the glass substrate towarda second edge of the glass substrate, for example in a range from about75% to about 25%, the spatial density being determined as a percentcoverage of a unit area. The spatial density of the deposited features(dots) can be substantially constant along a line parallel with thefirst edge.

In some embodiments the light guide may include a suitable underlyingsubstrate comprising a polymer layer deposited thereon. The underlyingsubstrate provides rigidity to the polymer layer, whereas the polymerlayer provides the necessary surface roughness characteristics. Theunderlying substrate may include any of the foregoing materials,including glass. The polymer layer may, in some examples, have athickness less than 100 micrometers. A surface texture may then beproduced in the exposed surface of the polymer layer such as by pressing(stamping) with a master mold (e.g. an etched glass plate produced bythe methods disclosed herein). In some embodiments the polymer layer maybe a film and the texture may be produced on the polymer layer prior toits application to the glass layer.

In aspects where a glass substrate is required to minimize electrostaticcharging, the etchant may be adjusted. For example, in some embodimentsthe etchant comprises acetic acid (HC₂H₃O₂) in a range from about 10% byweight to about 90% by weight, for example in a range from about 10% toabout 80% by weight, in a range from about 20% to about 70% by weight,in a range from about 20% by weight to about 30% by weight or in a rangefrom about 40% by weight to about 60% by weight. The etchant may furtherinclude NH₄F in a range from about 1% to about 50% by weight, forexample in a range from about 5% to about 40% by weight, in a range fromabout 10% by weight to about 30% by weight. In certain embodimentspolyethylene glycol or other organic solvents can be substituted for theacetic acid. Depending on the desired texture, etching times can be in arange from about thirty seconds to two minutes, and in some cases up tofour minutes. When the etching is complete, the glass substrate can besoaked in 1M H₂SO₄ for up to 1 minute to remove precipitated crystalresidue on the surface of the glass substrate. However, in someembodiments other mineral acids may be substituted for H₂SO₄, forexample HCl or HNO₃. In some cases hot water may be sufficient. A low pHvalue and/or a high temperature can increase the solubility of thecrystals. Finally, the glass substrate may be rinsed with water (e.g.deionized water) and dried. To avoid increased production costsassociated with high temperature etching, the etching process can beperformed at room temperature, with the understanding that highertemperature etching, if desired, falls within the scope of the method.

Additional features and advantages of the embodiments described hereinwill be set forth in the detailed description which follows, and in partwill be readily apparent to those skilled in the art from thatdescription or recognized by practicing the embodiments as describedherein, including the detailed description which follows, the claims, aswell as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description present embodiments of the presentdisclosure, and are intended to provide an overview or framework forunderstanding the nature and character of the embodiments as they areclaimed. The accompanying drawings are included to provide a furtherunderstanding of the embodiments, and are incorporated into andconstitute a part of this specification. The drawings illustrate variousembodiments of the present disclosure and together with the descriptionserve to explain the principles and operations of the embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional side view of a display device according toan embodiment described herein;

FIG. 2 is a top down view of the display device according to FIG. 1,showing a distribution of dots of a deposited material on a roughenedsurface;

FIG. 3 is a cross sectional side view of a lighting apparatus(backlight) comprising the display device of FIG. 1;

FIG. 4 is a plot showing modeled data of normalized power versusscattering angle for a textured surface having an RMS roughness of 50 nmand several correlation lengths;

FIG. 5 is a cross sectional side view of another embodiment of abacklight wherein the glass substrate comprising the backlight includesat least one angled edge;

FIG. 6 is a cross sectional side view of another display device whereinthe lighting apparatus comprises a fiber optic light source;

FIG. 7 is a cross sectional side view of a lighting apparatus comprisingthe display device of FIG. 6 wherein different wavelengths of light canbe coupled into the fiber optic light source;

FIG. 8 depicts a process flow diagram for a method of producing alighting apparatus according to an embodiment of the present disclosure;

FIG. 9 is a cross sectional side view of a glass substrate for use in abacklight as disclosed herein, wherein a protective laminate is appliedto a surface of the glass substrate;

FIG. 10 is a schematic diagram of an example nano-replication processthat can be used to produce a polymer film with a textured surface;

FIG. 11 is a drum used to produce a textured surface in the processshown in FIG. 10;

FIG. 12 is a perspective drawing of a projection system for projectingan image onto a transparent projection screen;

FIGS. 13A, 13B and 13C are edge views of several embodiments ofprojection screens useable with the projection system of FIG. 12;

FIG. 14 is a plot of luminance versus viewing angle for an examplebacklight etched under different etching conditions as described herein;

FIG. 15 is a plot showing luminance as a function of viewing angle for12 different backlights;

FIG. 16 is a plot illustrating the correlation between the correlationlength of the glass substrate surface texture and the viewing anglegoodness parameter;

FIG. 17 is a plot illustrating the correlation between a unit-lessporosity bi-modality parameter characterizing the glass substratesurface texture and the viewing angle goodness parameter;

FIG. 18 is a photomicrograph of a single dot deposited on a texturedsurface of a glass substrate (deposited feature);

FIG. 19 is another photomicrograph of another dot deposited on atextured surface of a glass substrate (deposited feature);

FIG. 20 is a plot showing the dimensions of the deposited feature ofFIG. 14;

FIG. 21 is a plot showing measured brightness as power scattered througha surface of the glass substrate in a region comprising a surfacetexture without dots, and a region comprising texture and dots;

FIG. 22 is a plot showing the relationship between glass substratetemperature during embodiments of the etching process described hereinand the viewing angle goodness parameter on two separate days;

FIGS. 23A, 23B and 23C are photomicrographs of a glass substrate surfaceshowing deposited crystalline precipitates resulting from an etchingprocess on the glass substrate surface;

FIG. 24 is a plot showing the effect of a conventional etching processon several different glasses;

FIG. 25 is a plot illustrating surface voltage reduction for glasssamples etched by different chemistries as a function of averageroughness R_(a);

FIG. 26A is a plot comparing surface roughness as a function of etchtime for glass samples wherein an acetic acid/ammonium fluoride etchantwas static and wherein the etchant was agitated;

FIG. 26B is a plot comparing surface roughness as a function of etchtime for glass samples wherein an polyethylene glycol/ammoniumbifluoride etchant was static and wherein the etchant was agitated;

FIG. 27A depicts haze as a function of R_(a) obtained through in situmask etching approach with a solution of 20% by weight NH₄F and 50% byweight acetic acid;

FIG. 27B plots haze as a function of R_(a) for glass etched in asolution of 11% by weight NH₄FHF and 25% by weight polyethylene glycol;

FIG. 28A through FIG. 28F are AFM plots of the surfaces for 6 samples C0through C5, respectively, wherein C0 is a control sample and C1 throughC5 are samples etched with various etchants and showing the surfacetexture of each surface.

FIG. 29 is a plot showing peak surface voltage resulting from theapplication of a peel test for four different etchants to samples C1-C5,and a control sample C0;

FIGS. 30A and 30B are a plots showing the degree of correlation betweenFOM (m=2, 6) and electrostatic charge-induced glass surface voltage forvarious topographical embodiments C0-C5;

FIGS. 31A and 31B are contour plots showing the effect of FOMcalculation parameters on the correlation coefficient R², i.e. theability to best correlate with electrostatic charge voltage response;

FIGS. 32A and 32B illustrate an idealized surface topographies and aprofile thereof (in arbitrary units), respectively, that assist indescribing the unique space wherein FOM and Ra (R_(q)) diverge;

FIGS. 33A and 33B illustrate another idealized surface topographies anda profile thereof (in arbitrary units), respectively, that assist indescribing the unique space wherein FOM and Ra (R_(q)) diverge;

FIGS. 34A and 34B illustrate still another idealized surfacetopographies and a profile thereof (in arbitrary units), respectively,that assist in describing the unique space wherein FOM and Ra (R_(q))diverge;

FIG. 35 is a graphical illustration of FOM vs. duty cycle for theidealized surface topographies of FIGS. 32A through 34B, the surfacehaving a constant roughness R_(a) (or R_(q)) and showing the relativedependence of FOM on surface area and independence from roughness;

FIG. 36 is a graphical illustration of FOM as a function of roughness(R_(a)) showing the independence of FOM from roughness; and

FIG. 37 is a schematic illustration showing electrostatic chargeresponse as a function of FOM and highlighting examples where roughness(R_(a) or R_(q)) is insufficient and/or doesn't uniquely describecontact-area/electrostatic charge reduction.

DETAILED DESCRIPTION

Conventional back light elements for illuminating visual display devicesare typically made using polymers such as polymethyl methacrylate (PMMA)or polycarbonate. Such materials are relatively inexpensive and offergood light transmission. As display devices become slimmer, however, thelight guides comprising the back light must also become thinner. Forexample, in some handheld devices these light guides are less than 1millimeter thick. Thin polymer light guides become expensive tofabricate. In addition, very thin polymer light guides degrade quicklyunder constant exposure to the light and heat emitted from the LEDsources during device use (e.g. photo-aging), especially near the lightcoupling zone where the LEDs are arrayed. Additionally, manufacturingthin warp-free polymer sheets is challenging, and exposure to anon-uniform temperature field during display operation can cause polymerlight guides to warp, leading to a defect called “pooling mura”.Polymers also have a low thermal conductivity, approximately 0.18 wattsper meter per Kelvin (W/m·K) for PMMA, and 0.19 W/m·K for polycarbonate.Consequently, polymer light guides have a reduced ability to dissipateheat buildup near the light coupling zone and can discolor and becomebrittle. Moreover, polymer light guides typically also display poorangular uniformity of luminance, such that the peak intensity of lightemitted from a light guide produced from materials like PMMA andpolycarbonate can occur at angles approaching approximately 70 degreesas measured from a normal to the light guide surface. Consequently,optical films are often utilized at the front surface of these lightguides to produce scattering and turn the light in the normal direction.Unfortunately, such optical films typically have poor transparency,making them unsuitable for use in a transparent display. As used herein,the front surface refers to the surface from which the light is emittedfrom the light guide for lighting purposes. For a liquid crystaldisplay, the front surface of the light guide, as used herein, is thesurface facing the LCD panel. Accordingly, the back side is the surfacefacing away from the display panel.

For at least the reasons above, alternatives that offer greaterrigidity, less warping, less discoloration and exhibit an optical losssufficiently low to produce viable light emitters are attractivealternatives. Material options include without limitation glass,sapphire, certain diamond-like materials or even calcium fluoride.

Glass, in particular, is not susceptible to photo-aging anddiscoloration under the prolonged light exposure conditions typical of adisplay device. Moreover, glass, particularly silica-based glass, can beproduced with very high thickness uniformity in exceptionallytransparent sheets at low cost compared to other materials that mightotherwise be suitable. As a result, glass can be an ideal light guide ina light emitter, and is particularly well suited for applications in aback light element for display devices. Accordingly, the followingdescription and examples present methods and articles based on glass,with the understanding that other materials may be used in themanufacture of a textured substrate having the characteristics, e.g.surface texture, disclosed herein.

In accordance with one or more embodiments disclosed herein, a“maskless” etching technique may be used to produce glass light guidesthat can be used in any number of applications, including as a lightguide for a backlight element in transparent or non-transparent displayapplications. The methodology involves a low cost, highly controllableprocess for producing a light scattering portion that includesnanometer-sized features distributed in and/or on a surface of a glasssubstrate. In addition, micrometer-sized light scatter suppressingfeatures may be deposited over the nanometer-sized features on the glasssurface. The combination of etched nanometer-sized features anddeposited micron-sized light scatter suppressing features tremendouslyimproves the light emitting properties of the light guide. In operation,light is coupled into and propagates through the glass substrate in awaveguide fashion and is incident on the light scattering surfaceportion. In response, the light scatters out of the glass structureacross a surface thereof in accordance with desirable opticalcharacteristics. However, where the light propagating through the glasssubstrate encounters a light scatter suppressing feature, total internalreflection may be restored, and the light continues propagating throughthe glass substrate without being scattered out. Thus, light extractionfrom the glass substrate via scattering is suppressed, and light outputmay thereby be controlled by controlling the distribution of lightscatter suppressing features (dots) on the surface of the glasssubstrate.

FIG. 1 shows a cross-sectional view of a textured light guide substrateas a dielectric slab waveguide illuminated from an edge thereof by anarray of light emitting diodes (LEDs). In the ray optics illustration ofFIG. 1, light rays emanate from the LEDs and propagate in the dielectricslab waveguide in a zigzag pattern, each ray reflecting alternativelyfrom the two major surfaces of the glass substrate. If the interfaces ofthe slab waveguide with the surrounding environment are perfectly smoothand the angle that each ray forms with an interface at which the ray isincident is greater than a critical angle φ_(c), relative to a normal tothat interface, the ray bounces off the interface at the same angle asthe incident angle and stays confined within the waveguide. Thisphenomenon is called total internal reflection.

However, total internal reflection breaks down when texture features atan interface of the light guide at the point of incidence are smallcompared to the wavelength of the incident light. That is, when thesurface of the light guide at the point of incidence is roughened andthe features comprising the texture are very small some light scatteringoccurs at each bounce of the light ray on the textured interface of thelight guide. Scattering produces light rays at angles different than theincident angle. Some of these rays do not satisfy the total internalreflection criterion and escape from the textured interface of the slabwaveguide and can illuminate the LCD panel at various angles. Some raysmay continue to reflect at or greater than the critical angle.

The methodologies described herein involve a wet chemical etchingprocess for texturing a glass substrate for illumination applicationsand articles made by that process that may be incorporated intosubsequent devices, such as the aforementioned display applications.Additionally, such treatments may be applied to glass substrates forother purposes, such as suppressing electrostatic charging of substratesduring processing of such substrates.

Creating texture on glass surfaces using fluoride-containing solutionsrequires an etch mask, since without a mask amorphous homogenoussilicate glass tends to etch evenly on a scale larger than the molecularlevel, reducing a thickness of the glass, but without creating texture.Many methods have been proposed for masking glass etching to providepatterned textures for various applications. Such methods can be dividedinto those requiring a separate masking process prior to etching andthose which form a mask in situ during etching, so-called “maskless”etching since there is no mask prior to the start of the etching. Forthe purposes of this disclosure, a mask can be considered any materialthat provides a barrier to etching, and may be applied to a glasssurface with various lateral feature sizes and with various levels ofdurability and adhesion to the glass.

Many methods of mask application, such as ink jet printing, havelimitations on the scale of the mask that can be applied in that they donot enable the deposition of small, nanometer-sized features. In fact,most methods produce textures that are on a micrometer scale in bothlateral feature size and depth of etch and so create a visible “frosted”appearance to glass that reduces transparency, increases haze, anddecreases glare and surface reflectivity.

In situ masking and glass etching involves a complex process of maskformation from byproducts of glass dissolution plus etchant.Precipitates that form (sometimes crystalline) are often somewhatsoluble in the etchant, making modeling of this process difficult.Moreover, creating a differential etch using maskless etching mayinvolve multiple steps to create the mask by contact with a frostingsolution or gel, and subsequent steps to remove the mask and etchant. Insitu etch masks can also produce various textures depending on theiradhesion to the substrate and durability in the wet etchant, and it canbe shown that less durable masks result in shallower textures. Depth ofetch is also determined by the size of the mask features, with smallerfeatures unable to support deeper etch profiles because maskundercutting occurs more readily. Therefore, mask chemistry, glasschemistry, and etch chemistry should all be considered when formingnanometer-scale textures.

Disclosed herein is a glass substrate with a textured surface suitableas a light guide plate for a light emitter with a broad angulardistribution of luminance, and a process for making the same utilizing amodified maskless etch method. The method produces texture on a glasssurface that is useful for a display light guide applications and anyother applications where controlled illumination is desired, and inparticular a transparent source of light. Methods disclosed herein mayalso be used to produce textured surfaces that reduce electrostaticcharging without a noticeable formation of haze.

The present disclosure describes substrates and light guides thatenhance thermal stability, have a relatively low coefficient of thermalexpansion and resistance to thermal breakdown, can produce uniformluminance over a broad viewing angle range with high brightness, andexhibit transparency with low haze. The surface texture properties thathave been observed and can be responsible for these desirable opticalproperties may include a correlation length of the texture featuresequal to or less than 150 nm and a root mean square (RMS) roughness(R_(q)) of the textured surface in a range from about 5 nm to about 75nanometers, for example in a range from about 5 nanometers to about 40nm, in a range from about 5 nanometers to about 30 nm, and in a rangefrom about 5 nanometers to about 20 nm. Generally, the smaller the valueof the RMS roughness, the better the performance of the light guide interms of its scattering characteristics, as the influence of roughnesson physical quantities can depend, inter alia, on the correlationproperties and/or spatial frequencies present on the surface. The notionof a correlation length is introduced, the correlation length beingrelated to the autocorrelation function G_(x)(τ_(x)) of a surfaceexpressed as a one-dimensional function z(x), where

$\begin{matrix}{{{G_{x}\left( \tau_{x} \right)} = {\lim_{L\rightarrow\infty}{\frac{1}{2L}{\int_{- L}^{L}{{z(x)}{z\left( {x + \tau_{x}} \right)}{dx}}}}}},} & (1)\end{matrix}$where L represents one half the profile length of the sample scan(distance over which the autocorrelation function is determined), z isthe height of the surface relative to a mean height, x represents aposition on the surface in the x direction, τ is the distance betweentwo positions x₁ and x₂. Sampling of the surface obtains an array ofheights z(j) across M positions spaced Δx apart. Here, k is an integerfrom 0 to M−1. Approximating for τ_(x)=kΔ_(x), equation (1) can bereduced to equation (2) below,

$\begin{matrix}{G_{k} = {\frac{1}{M - k}{\sum\limits_{j = 0}^{M - 1 - k}{z_{j}z_{j + k}}}}} & (2)\end{matrix}$

The autocorrelation function describes how the surface is correlated toitself at a distance τ_(x). The autocorrelation function goes to zerofor large τ_(x) such that the heights of the surface features becomeindependent. Thus, one can refer to the autocorrelation length T, orsimply the correlation length to represent the characteristic lateralfeature size of a surface. To obtain the correlation length frommeasured data, one may select, for example, a Gaussian model forG_(x)(τ_(x)), for example of the form,

$\begin{matrix}{{{G_{x}\left( \tau_{x} \right)} = {\sigma^{2}{\exp\left( {- \frac{\tau_{x}^{2}}{T^{2}}} \right)}}},} & (3)\end{matrix}$calculate a discretized G_(k) from measured data (using, for example, afast Fourier transform), and fit the model on the discretizedautocorrelation function. Alternative approaches may use an exponentialmodel. Calculations used herein employed a Gaussian model and allcorrelations lengths are expressed as a Gaussian correlation lengthunless otherwise stated. In other embodiments the calculations above maybe altered to include a two-dimensional consideration of the surfacerather than a one dimensional approach. Software programs such asGwyddion, an open-source software platform developed and supported bythe Department of Nanometrology, Czech Metrology Institute intended foranalysis of height fields obtained by scanning probe microscopytechniques to calculate an autocorrelation function are available thatcan compute the correlation length based on measured data.

These chemical etch methods can be divided into three areas needed tomake specific textures resulting in good light guide opticalproperties: 1) etchant chemistry, 2) glass chemistry, and 3) processsteps.

As described supra, the chemical etch process includes etching the glasssubstrate in an etchant, for example an etchant bath, that comprises amixture of an organic solvent such as acetic acid (AA) and an inorganicacid such as ammonium fluoride (NH₄F). The glass structure resultingfrom exposure to the etchant is a textured glass substrate. Forillumination applications, the textured glass substrate may be optimizedby controlling parameters of the process, such as but not limited to thecomposition of the etchants, the etching time, the etchant temperatureand the glass temperature. A reliance on the addition of alkali oralkaline earth salts for removal of the mask is unnecessary.

Other additives may provide advantages as well. These may include: A dyeto add color to the etchant and enable a visual aid for rinsing (commonfood-grade dye can suffice), and viscosity modification components tothicken the acid and enable painting or spraying the etchant on glass asopposed to dipping. A thickened acid can also prove advantageous sincethis is likely to reduce the vapor pressure and thereby reduce defectscaused by acid vapor contact with the substrate. For example, a suitablethickening agent is polycaprolactone, a synthetic polyester which meltsat 60° C. and which can be used to make a wax etchant. Polycaprolactoneis soluble in acetic acid and insoluble in water.

The process for modifying a glass substrate with an etched texture on atleast one surface thereof may in some embodiments comprise at least sixsteps, all of which can be performed within a temperature range fromabout 18° C. to about 22° C. (it should be noted that acetic acid beginsto freeze at temperatures below approximately 17° C. Accordingly, aminimum temperature should be greater than 17° C.).

In a first step of an example process, the glass substrate to be etchedis cleaned using a detergent to remove all inorganic contamination, thenrinsed sufficiently to remove detergent residue. A level of cleanlinesssufficient to obtain a water contact angle of less than about 20° shouldbe attained. Contact angle can be evaluated using, for example, a DSA100drop shape analyzer manufactured by Kruss GmbH and employing a sessiledrop method, which was done for experiments described herein, althoughother suitable methods may also be used. In an optional second step ofthe example process the back side of the glass substrate can belaminated with a suitable adhesive polymer film (or other acid barrier),if only a single side of the glass substrate is to be etched. Thepolymer film can be removed from the glass substrate after the etchingprocess is completed. It should be noted that a similar process can beused if the opposite (back side) of the substrate is to be etched,wherein the polymer film is added to the front surface of thesubstrate.)

In a third step of the example process, the glass substrate is contactedwith an etchant for a time sufficient to create the desired texture(typically in a range from about 0.5 minutes to 6 minutes). Forimmersion, fast insertion and suitable environmental controls, forexample an ambient air flow of at least a 2.83 cubic meter per minute inthe enclosure in which the etching occurs may be used to limit exposureof the glass substrate to acid vapor prior to and/or during insertion.The glass substrate should be inserted into the acid bath using a smoothmotion to prevent defects forming in the etched surface. The glasssubstrate should be dry prior to contact with the etchant.

In a fourth step, the glass substrate is removed from the etchant andallowed to drain, then rinsed one or more times with water, for exampledeionized water, or alternatively with a solution in which theprecipitant is dissolvable. Agitation of either the glass substrate orthe rinse liquid sufficient to ensure uniform diffusion offluoride-containing acid clinging to the glass substrate may beperformed. Rinse steps can employ agitation to prevent defects. Smalloscillations of approximately 300 oscillations per minute aresufficient, for example between about 250 and 350 oscillations perminute. The etchant bath may in some instances be recirculated toprevent stratifications and depletion.

In a fifth step of the example process any etchant-blocking filmpreviously applied to the back side of the glass substrate can beremoved, such as by peeling.

In a sixth step of the example process, the glass substrate can be driedusing forced clean (filtered) air to prevent water spots, or spots fromother rinsing solutions, from forming on the glass substrate.

The example process outlined briefly above is capable of providing thespecific textures and optical properties described herein, and also toenable high uniformity of etch for each sample when combined withfeatures from the following detailed description.

With reference to FIG. 1, not shown to scale, an edge view of a displaydevice 10 is depicted comprising a display panel 12, such as an LCDdisplay panel, and a light emitter 14 in accordance with one or moreembodiments described herein. Light emitter 14 may be employed toprocess light for a display system or other application.

In reference to FIGS. 1 and 2, light emitter 14 comprises a light source16 and a glass substrate 18 that serves as a light guide. Light coupledinto glass substrate 18 from light source 16 propagates within the glasssubstrate and is scattered at a major surface of glass substrate 18 anddirected, in the present figure, through an opposite surface of glasssubstrate 18, as indicated by arrows 20. Glass substrate 18 comprisesfirst and second spaced apart major surfaces 22, 24. First and secondmajor surfaces 22 and 24 may in some examples be parallel with oneanother. Glass substrate 18 may have a rectangular shape with a firstedge 26 and a second edge 28 opposite to and parallel with first edge26. First and second major surfaces 22, 24 extend between first andsecond edges 26, 28.

For purposes of description and not limitation, the distance betweenfirst and second edges 26, 28, as best seen with the aid of FIG. 2, willbe referred to as the length L of glass substrate 18. Accordingly, glasssubstrate 18 further comprises a width W extending between third andfourth edges 30 and 32, wherein third edge 30 is parallel with fourthedge 32, and both third and fourth edges 30, 32 are generallyperpendicular to and intersect with first and second edges 26 and 28.

Glass substrate 18 may be any suitable glass that can withstand theprocessing parameters expressly or inherently disclosed herein may beemployed as glass substrate 18, for example an alkali silicate glass, analuminosilicate glass, or an aluminoborosilicate glass. The glassmaterial may be a silica-based glass, for example code 2318 glass, code2319 glass, code 2320 glass, Eagle XG® glass, Lotus™, and soda-limeglass, etc., all available from Corning, Inc. Other display-type glassesmay also benefit from the processes described herein. Thus, glasssubstrate 18 is not limited to the previously described CorningIncorporated glasses. For example, one selection factor for the glassmay be whether a subsequent ion exchange process may be performed, inwhich case it is generally desirable that the glass be analkali-containing glass.

Display glass substrates can have various compositions and be formed bydifferent processes. Suitable formation processes include, but are notlimited to float processes and downdraw processes such as slot-draw andfusion draw processes. See, for example, U.S. Pat. Nos. 3,338,696 and3,682,609. In the slot-draw and fusion draw processes, the newly-formedglass sheet is oriented in a vertical direction. One glass substrate,Lotus™, manufactured by Corning, Inc., has a small coefficient ofthermal expansion and is superior in dimensional stability andworkability at relatively high processing temperatures. Lotus™ glasscontains little, if any, alkali components in the glass.

Suitable glass display substrates include high performance glasssubstrates manufactured by Corning, Inc. The glass substrates arespecifically designed to be used in the manufacture of flat paneldisplays and exhibit densities of less than 2.45 g/cm³ and a liquidusviscosity (defined as the viscosity of the glass at the liquidustemperature) greater than about 200,000 poises, or greater than about400,000 poises, or greater than about 600,000 poises, or greater thanabout 800,000 poises. Additionally, suitable glass substrates exhibitsubstantially linear coefficients of thermal expansion over thetemperature range of 0° to 300° C. of 28-35×10⁻⁷/° C., or of28-33×10⁻⁷/° C., and strain points higher than about 650° C. As used inthis specification and the appended claims, the term “substantiallylinear” means that the linear regression of data points across thespecified range has a coefficient of determination greater than or equalto about 0.9, or greater than or equal to about 0.95, or greater than orequal to about 0.98 or greater than or equal to about 0.99, or greaterthan or equal to about 0.995. Suitable glass substrates include thosewith a melting temperature less than 1700° C. In addition, suitableglass substrates may exhibit a weight loss of less than 0.5 mg/cm² afterimmersion in a solution of 1 part HF (50 wt. %) and 10 parts NH₄F (40wt. %) for 5 minutes at 30° C.

In one embodiment of the described process, the glass substrate has acomposition in which the major components of the glass are SiO₂, Al₂O₃,B₂O₃, and at least two alkaline earth oxides. Suitable alkaline earthoxides include, but are not limited to MgO, BaO and CaO. The SiO₂ servesas the basic glass former of the glass and has a concentration greaterthan or equal to about 64 mole percent in order to provide the glasswith a density and chemical durability suitable for a flat panel displayglass, e.g., a glass suitable for use in an active matrix liquid crystaldisplay panel (AMLCD), and a liquidus temperature (liquidus viscosity)which allows the glass to be formed by a downdraw process (e.g., afusion process) described in more detail below. Suitable glasssubstrates have a density less than or equal to about 2.45 grams/cm³, orless than or equal to about 2.41 grams/cm³, a weight loss which is lessthan or equal to about 0.8 milligrams/cm² when a polished sample isexposed to a 5% HCl solution for 24 hours at 95° C., and a weight lossof less than 1.5 milligrams/cm² when exposed to a solution of 1 volumeof 50% by weight HF and 10 volumes 40% by weight NH₄F at 30° C. for 5minutes.

Suitable glass for use with embodiments of the present disclosure havean SiO₂ concentration less than or equal to about 71 mole percent toallow batch materials to be melted using conventional, high volumemelting techniques, e.g., Joule melting in a refractory melter. Indetailed embodiments, the SiO₂ concentration is in a range from about66.0 to about 70.5 mole percent, or in a range from about 66.5 to about70.0 mole percent, or in a range from about 67.0 to about 69.5 molepercent. As a result of the SiO₂ content, suitable glasses may havemelting temperatures equal to or greater than about 1600° C.

Aluminum oxide (Al₂O₃) is another glass former suitable for use withembodiments of the disclosure. Without being bound by any particulartheory of operation, it is believed that an Al₂O₃ concentration equal toor greater than about 9.0 mole percent provides a glass with a lowliquidus temperature and a corresponding high liquidus viscosity. Theuse of at least about 9.0 mole percent Al₂O₃ may also improve the strainpoint and the modulus of the glass. In detailed embodiments, the Al₂O₃concentration may be in the range from about 9.5 to about 11.5 molepercent.

Boron oxide (B₂O₃) is both a glass former and a flux that aids meltingand lowers the melting temperature. To achieve these effects, glassesfor use with embodiments of the present disclosure can have B₂O₃concentrations equal to or greater than about 7.0 mole percent. Largeamounts of B₂O₃, however, lead to reductions in strain point(approximately 10° C. for each mole percent increase in B₂O₃ above 7.0mole percent), modulus, and chemical durability.

Suitable glass substrates may have a strain point equal to or greaterthan about 650° C., equal to or greater than about 655° C., or equal toor greater than about 660° C., a Young's modulus equal to or greaterthan 10.0×10⁶ psi, and a chemical durability as described above inconnection with the discussion of the SiO₂ content of the glass. Withoutbeing bound by any particular theory of operation, it is believed that ahigh strain point may help prevent panel distortion due tocompaction/shrinkage during thermal processing subsequent tomanufacturing of the glass. Accordingly, it is believed that a highYoung's modulus may reduce the amount of sag exhibited by large glasssheets during shipping and handling.

In addition to the glass formers (SiO₂, Al₂O₃, and B₂O₃), suitable glasssubstrates may also include at least two alkaline earth oxides, i.e., atleast MgO and CaO, and, optionally, SrO and/or BaO. Without being boundby any particular theory of operation, it is believed that alkalineearth oxides provide the glass with various properties important tomelting, fining, forming, and ultimate use. In some embodiments, the MgOconcentration is greater than or equal to about 1.0 mole percent. Indetailed embodiments, the MgO concentration is in the range of about 1.6and about 2.4 mole percent.

Of the alkaline earth oxides, the CaO concentration of some embodimentsof the glass substrate is the largest. Without being bound by anyparticular theory of operation, it is believed that CaO produces lowliquidus temperatures (high liquidus viscosities), high strain pointsand moduli, and coefficients of thermal expansion (CTE's) in the mostdesired ranges for flat panel applications, specifically, AMLCDapplications. It is also believed that CaO contributes favorably tochemical durability, and compared to other alkaline earth oxides, CaO isrelatively inexpensive as a batch material. Accordingly, in someembodiments, the CaO concentration is greater than or equal to about 6.0mole percent. In specific embodiments, the CaO concentration in thedisplay glass is less than or equal to about 11.5 mole percent, or inthe range of about 6.5 and about 10.5 mole percent.

In some examples, the glass may comprise 60-70 mol % SiO₂; 6-14 mol %Al₂O₃; 0-15 mol % B₂O₃; 0-15 mol % Li₂O; 0-20 mol % Na₂O; 0-10 mol %K₂O; 0-8 mol % MgO; 0-10 mol % CaO; 0-5 mol % ZrO₂; 0-1 mol % SnO₂; 0-1mol % CeO₂; less than 50 ppm As₂O₃; and less than 50 ppm Sb₂O₃; wherein12 mol % Li₂O+Na₂O+K₂O≤20 mol % and 0 mol %≤MgO+CaO≤10 mol %, andwherein the silicate glass is substantially free of lithium.

Certain glass substrates described herein can be what is referred to inthe art as laminated glass. In one aspect, the display glass substrateis produced by fusion drawing a glass skin to at least one exposedsurface of a glass core. Generally, the glass skin will possess a strainpoint equal to or greater than 650° C. In some embodiments, the skinglass composition has a strain point equal to or greater than 670° C.,equal to or greater than 690° C., equal to or greater than 710° C.,equal to or greater than 730° C., equal to or greater than 750° C.,equal to or greater than 770° C., or equal to or greater than 790° C.The strain point of the disclosed compositions can be determined by oneof ordinary skill in the art using known techniques. For example, thestrain point can be determined using ASTM method C336.

In some embodiments, the glass skin can be applied to an exposed surfaceof a glass core by a fusion process. An example of a suitable fusionprocess is disclosed in U.S. Pat. No. 4,214,886, which is incorporatedby reference herein in its entirety. The fusion glass substrate formingprocess can be summarized as follows. At least two glasses of differentcompositions (e.g., the base or core glass sheet and the skin) areseparately melted. Each of the glasses is then delivered through anappropriate delivery system to a respective overflow distributor. Thedistributors are mounted one above the other so that the glass from eachflows over top edge portions of the distributor and down at least oneside to form a uniform flow layer of appropriate thickness on one orboth sides of the distributor. The molten glass overflowing the lowerdistributor flows downwardly along the distributor walls and forms aninitial glass flow layer adjacent to the converging outer surfaces ofthe bottom distributer. Likewise, molten glass overflowing from theupper distributor flows downwardly over the upper distributor walls andflows over an outer surface of the initial glass flow layer. The twoindividual layers of glass from the two distributers are broughttogether and fused at the draw line formed where the converging surfacesof the lower distributor meet to form a single continuously laminatedribbon of glass. The central glass in a two-glass laminate is called thecore glass, whereas the glass positioned on the external surface of thecore glass is called the skin glass. A skin glass can be positioned oneach surface of the core glass, or there may be only one skin glasslayer positioned on a single side of the core glass. When just one skinglass is fused directly to the core, the skin is “adjacent” to the core.

The overflow distributor process provides a fire polished surface to theglass ribbon so formed, and the uniformly distributed thickness of theglass ribbon provided by the controlled distributor(s), and the glasssheets cut therefrom, provides the glass sheets with superior opticalquality. The glass sheets used as display glass substrates can have athickness in the range of 100 micrometers (μm) to about 0.7 μm, butother glass sheets that may benefit from the methods described hereinmay have a thickness in a range from about 10 μm to about 5 mm. Otherfusion processes, which can be used in the methods disclosed herein, aredescribed in U.S. Pat. Nos. 3,338,696, 3,682,609, 4,102,664, 4,880,453,and U.S. Published Application No. 2005/0001201, which are incorporatedby reference herein in their entireties. The fusion manufacturingprocess offers advantages for the display industry, including glass thatis flat with excellent thickness control and glass that has a pristinesurface quality and scalability. Glass substrate flatness can beimportant in the production of panels for liquid crystal display (LCD)televisions as any deviations from flatness can result in visualdistortions.

In some embodiments, the glass substrate will possess a strain pointequal to or greater than 640° C., a coefficient of thermal expansion ina range from about 31×10⁻⁷/° C. to about 57×10⁻⁷/° C., a weight lossless than 20 mg/cm² after immersion for 24 hours in an aqueous 5% byweight HCl solution at about 95° C., that is nominally free from alkalimetal oxides and has a composition, calculated in weight percent on theoxide basis, comprising about 49 to 67% SiO₂, at least about 6% Al₂O₃,SiO₂+Al₂O₃>68%, B₂O₃ in a range from about 0% to about 15%, at least onealkaline earth metal oxide selected from the group consisting of, in thepreparations indicated, about 0 to 21% BaO, about 0 to 15% SrO, about 0to 18% CaO, about 0 to 8% MgO and about 12 to 30% BaO+CaO+SrO+MgO.

In accordance with embodiments disclosed herein and shown in a close upview in FIG. 3, at least one surface of glass substrate 18, e.g. firstmajor surface 22, is treated to include a plurality of nanometer-scalelight scattering elements 34. As will be shown below, the plurality oflight scattering elements 34 produce a texture on first major surface 22and the texture can be formed by a modification of first major surface22 through the action of etching with an etchant as described supra anddisclosed in greater detail below.

A light ray 36 entering glass substrate 18 through first edge 26propagates through the glass substrate under conditions of totalinternal reflection in a direction generally along axis 38 until the rayof light impinges on scattering elements 34. For example, if the angle φbetween a normal 40 to a surface of the glass substrate is greater thanthe critical angle φ_(c), where φ_(c) can be calculated according toequation (4) below,φ_(c)=arcsin(n ₂ /n ₁)  (4)where n₁ is the index of refraction of glass substrate 18 and n₂ is theindex of refraction of air, the light propagating in glass substrate 18and incident at the glass-air interface will be totally internallyreflected.

However, when the propagating light is incident on a textured surface atleast a portion of the incident light is scattered, as shown by arrows20 (FIG. 1). At least some of the scattered light can be directed out ofthe glass substrate. If the glass substrate is to be used as a lightsource (e.g. a back light element) for an LCD display device, glasssubstrate 18 can be positioned such that the emitted light is directedthrough an adjacent LCD panel 12. The textured surface may be the firstmajor surface 22 farthest from display panel 12 or the textured surfacemay be second major surface 24 closest to display panel 12. In someinstances, both first and second surfaces may be textured. The opticalcharacteristics of glass substrate 18 are controllable inter alia, viathe process for producing the scattering elements 34. FIG. 4 depictsmodeled data showing normalized power as a function of scattering anglefor a surface, e.g. first major surface 22, comprising an RMS roughness(R_(q)) of approximately 50 nanometers (nm). Curve 50 represents asurface wherein a correlation length of the scattering elements is 200nanometers, while curve 52 represents a correlation length of 150 nm andcurve 54 represents a correlation length of 100 nanometers. The datashow a trend toward increased forward scattering (reduced scatteringangle) as the correlation length is reduced. Textured surfaces producedaccording to the present disclosure can provide a glass substrate thatdoes not create a visibly frosted appearance to the glass. Such afrosted appearance reduces transparency and increases haze. The surfacesresulting from the treatments described herein have been shown toproduce Distinctness of Image (DOI) values equal to or greater than 90%.

It has been found that the sizes of the plurality of light scatteringelements 34 will affect the light scattering properties of light emitter14. For example, the textured first major surface 22 including lightscattering elements 34 can comprise a root mean square (RMS) roughnessR(_(q)) in a range from about 5 nanometers to about 75 nanometers, forexample in a range from about 20 nanometers to about 60 nanometers, andin other examples in a range from about 20 nanometers to about 35nanometers, or from about 20 nanometers to about 30 nanometers. In otherexamples, a root mean square (RMS) roughness R(_(q)) in a range fromabout 5 nanometers to about 20 nanometers, or in a range from about 5nanometers to about 15 nanometers can be attained. A correlation lengthT of the scattering elements can be equal to or less than about 252 nm,for example equal to or less than 200 nm, equal to or less than 150 nm,or equal to or less than 100 nm. As noted previously, it has been foundthat a correlation length equal to or less than about 100 nm isparticularly advantageous when considered in the context of a viewingangle goodness parameter, described in more detail herein. Thecorrelation length may be found, for example, by measuring a roughnessof the treated surface via a scanning probe microscopy technique such asatomic force microscopy (AFM), and using a software computationalpackage, like Gwyddion.

Light source 16 may be one or a plurality of LEDs, or other suitablelight generating elements. Moreover, while FIG. 2 illustrates lightsource 16 along only a single edge or edge surface of glass substrate18, a light source 16 may be disposed along one or more edge surfaces orone or more borders of glass substrate 18. In one or more embodiments,the one or more edge surfaces of glass substrate 18 may be beveled, asshown in FIG. 5, and may include a metalized reflective surface. Thebevel angle γ is chosen to redirect any light propagating within theglass substrate 18 in one or more directions that reduce the escape oflight out of the at least one edge surface. Light emitter 14 mayoptionally include a light redirecting (or blocking) border 40 toimprove the optical appearance near the edges of the glass substrate 18.

One or more alternative embodiments may employ one or more light sources16 and associated structures of the type(s) disclosed in PCT PublicationNo. WO12/058084 (PCT/US11/57032), published on May 3, 2012, the entiredisclosure of which is incorporated herein in its entirety by reference.

FIG. 6 illustrates a light emitter 14 wherein light source 16 comprisesat least one light diffusing fiber 16 a extending along first edge 26 ofglass substrate 18. The light diffusing fiber 16 a may be on the orderof about 250-300 microns in diameter. Light extracted from lightdiffusing fiber 16 a, such as by scattering, can be coupled intosubstrate 18.

In another embodiment shown in FIG. 7, one or more laser sources mayproduce white (or semi-white) light to couple into light diffusing fiber16 a and thereafter into substrate 18. In one or more alternativeembodiments, one or more laser sources, such as a red laser source(RED), a green laser source (GREEN) and a blue laser source (BLUE), maycouple light energy of differing wavelengths into a single lightdiffusing fiber 16 a, or multiple light diffusing fibers, in a way thatcouples such light into glass substrate 18 and causes diffusion andscattering as discussed previously. Using multiple laser sources permitsthe production of any number of colors by adjusting the power level ofeach laser source.

Further details concerning various structures and methodologiesassociated with modulating the laser sources (e.g., time sequentialmodulation) to achieve desirable color image functionality and otherdetails concerning the use of the light diffusing fiber 16 a may befound in: U.S. Patent Publication No. US2012/0275745A1, published onNov. 1, 2012; U.S. Patent Publication No. US2013/0272014A1, published onOct. 17, 2013; and; U.S. Patent Publication No. US2014/0092623A1,published on Apr. 3, 2014, the entire disclosures of which areincorporated herein by reference.

Reference is now made to FIG. 8, a flow chart illustrating exampleprocess steps in connection with producing light emitter 14, andvariants thereof, in accordance with embodiments disclosed herein.

At step 100, glass substrate 18 may, if necessary, be processed to makethe glass substrate ready for the subsequent etching step. Inparticular, glass substrate 18 may be ground and polished, if necessary,to achieve a desirable thickness and/or flatness and/or otherwise sized,and thereafter cleaned and washed in step 102. Step 100 may include edgedressing of substrate 18, for example edge/edge surface beveling.Washing may be accomplished, for example, by exposing first majorsurface 22, alternatively second major surface 24, or both first andsecond major surfaces 22, 24, to a suitable detergent cleaning solutionsuch as Semiclean KG, with or without ultrasonic agitation, and multiplede-ionized rinsing steps, followed by drying of the glass substrate. Forexample, the glass substrate may be placed in a bath comprising thecleaning solution, then rinsed by dipping or spraying with water. Theglass substrate should be cleaned sufficiently such that a water contactangle of the surface to be etched is equal to or less than about 20degrees.

At step 104, if a major surface of the glass substrate 18 will not beetched, for example second major surface 24, the major surface not to beetched may optionally be laminated (preferably over the entire surface)with a protective film 42 that resists or prevents etching (see FIG. 9).It should be apparent that should it be desired to etch both majorsurfaces of the glass substrate with the etchant, a protectivelaminating film may not be required. In another optional next step 106,the major surface or surfaces to be etched, for example first majorsurface 22, may be subjected to a process for removing contamination,such as adsorbed organic contamination. Such process may includeexposing at least first major surface 22 to a plasma cleaning process asis known in the art.

At step 108, the major surface of glass substrate 18 to be etched issubjected to etching by exposing the surface to an etchant comprising anorganic solvent, for example acetic acid (AA), and an inorganic acid,such as ammonium fluoride (NH₄F), for a time and at a temperaturesufficient to obtain the requisite roughness and correlations lengths.The etchant may comprise an organic solvent, for example acetic acid, ina range from about 10% by weight to about 98% by weight. In furtherexamples, the organic solvent may be in a range from about 10% to 80% byweight, in a range from about 20% to about 70% by weight, or in a rangefrom about 30% to about 60% by weight. In still other examples theorganic solvent may be in a range from about 80% by weight to about 98%by weight, for example in a range from about 90% by weight to about 98%by weight or in a range from about 92% by weight to about 98% by weight.

The etchant may further comprise an inorganic acid in a range from about0.5% by weight to about 50% by weight. For example, the etchant maycomprise an inorganic acid in a range from about 10% by weight to about40% by weight, from about 15% by weight to about 30% by weight. In otherexamples the etchant may comprise an inorganic acid in an amount fromabout 0.5% to about 10% by weight, or from about 0.5% by weight to about6% by weight. In certain embodiments the etchant may contain water (e.g.de-ionized water) in an amount equal to or less than 8% by weight, forexample in a range from about 0.5% by weight to about 6.0% by weight,from about 0.5% by weight to about 4.0% by weight, or from about 0.5% byweight to about 2.0% by weight. Optimal concentrations may varydepending on glass composition and environmental conditions, such as dewpoint, which can affect physisorbed water on the glass surface, and theresultant texture desired.

In some embodiments certain other additives may be incorporated into theetchant. For example, a dye may be added to the etchant to add color andproduce a visual aid for rinsing. In addition, as previously described,viscosity modification components may be added to increase the viscosityof the etchant and enable contact by slot, slide or curtain coatingetchant on glass vs. dipping, to provide a light emitter with uniformappearance. Alternatively, artistic effects can be achieved for whennon-uniform light extraction is required by applying etchant usingspraying or painting techniques. A high viscosity etchant may reduce thevapor pressure and thereby reduce vapor-induced defects. The viscosityof the etchant can be adjusted to be compatible with the selected methodof application as needed. Suitable polymers such as polycaprolactonethat are soluble in acetic acid may be used to modify the rheology ofthe etch solutions.

The etch time may range from about 30 seconds to about 10 minutes,although other times may be employed if such other times yield desirableresults. For example, exposure times in a range from about 1 minute toabout 4 minutes have been found useful for many glass compositions. Thetemperature of the etchant may be in a range from about 18° C. to about90° C., for example in a range from about 18° C. to about 40° C., in arange from about 18° C. to about 35° C., in a range from about 18° C. toabout 30° C., in a range from about 18° C. to about 25° C., or even in arange from about 18° C. to about 22° C. Etchant temperatures in thelower ranges, for example, ranges in the 18° C. to 30° C. range, arefavored since this can reduce vapor pressure and produces fewervapor-related defects on the glass. Again, the etch time and etchanttemperature will have an effect on the feature size, shapes, anddistribution of the resulting light scattering elements 34, which arenanometer-scale protrusions extending from the etched surface of theglass substrate and produced by the etching process. Etch texture canvary with glass composition. Accordingly, etchant recipes optimized forone glass composition may require modification to remain effective forother compositions. Such modification is typically accomplished throughexperimentation.

In some embodiments, as best seen in FIG. 9, if a surface is not to beetched, e.g. second major surface 24, that surface may be protected byapplying an etchant-resistant protective film 42 to the surface.Etchant-resistant protective film 42 may be removed after the etchingstep(s). Table 1 below presents non-limiting examples of some suitableetchant solutions and etch times.

TABLE 1 Weight % etch components Etch Solution Ammonium Time, numberAcetic Acid Fluoride Water min 1 92.8 5.4 1.8 2.5 2 94.1 4.1 1.8 3.5 395.8 1.4 2.9 4.1 4 93.4 3.8 2.8 5.1 5 94.9 1.1 4.0 5.5 6 92.1 2.2 5.85.5 8 91.0 8.0 1.0 9.0 9 91.0 1.0 8.0 9.0 10 91.0 4.5 4.5 7.0

Thus, the etchant should include an organic solvent, e.g. acetic acid,in a concentration from about 90.0 percent by weight to about 96 percentby weight, and ammonium fluoride in a concentration in a range fromabout 1.0 percent by weight to about 8.0 percent by weight. The etchantmay include water in an amount equal to or less than about 8.0 percentby weight, for example from about 1.0 percent by weight to about 6.0percent by weight. In addition, it has been found that the temperatureof the glass substrate itself at the time the glass substrate is exposedto the etchant can in some instances also affect etching results.Accordingly, the glass substrate when exposed to the etchant may be at atemperature in a range from about 20° C. to about 60° C., for example ina range from about 20° C. to about 50° C., or in a range from about 30°C. to about 40° C. (see Example 3). The optimal temperature will dependon glass type, environmental conditions and the desired texture. Etchtimes may extend from about 1 minute to about 10 minutes, although otheretch times as may be needed to achieve the desired surface texture mayalso be used. As will be discussed further below, the foregoingchemistry may be adjusted to accommodate different applications anddifferent glass compositions.

At step 110, glass substrate 18 can be rinsed with a rinsing solution,for example water, in one or more rinsing (de-acidifying) actions.Alternatively, in some embodiments the glass substrate can be soaked in1M H₂SO₄ for up to 1 minute to remove precipitated crystal residue onthe surface of the glass substrate. However, other mineral acids may besubstituted for H₂SO₄, for example HCl or HNO₃. The rinsing solution maybe heated in one or more of the rinsing actions. In some embodiments therinsing solution may include other fluids in which precipitant from theetching process is dissolvable.

At an optional step 112, the glass substrate may be subject to an ionexchange process. If such a process is desired, then the process canbegin with preparing at step 100 a glass substrate 18 capable of beingion exchanged. For example, ion-exchangeable glasses suitable for use inembodiments described herein include without limitation alkalialuminosilicate glasses or alkali aluminoborosilicate glasses, thoughother glass compositions may be substituted. As used herein, beingcapable of ion exchange means a glass capable of exchanging cationslocated at or near the surface of the glass substrate 18 with cations ofthe same valence that are either larger or smaller in size.

The ion exchange process is carried out by immersion of the glasssubstrate 18 into a molten salt bath for a predetermined period of time,where ions within the glass substrate at or near the surface thereof areexchanged for larger metal ions, for example, from the salt bath. By wayof example, the molten salt bath may include potassium nitrate (KNO₃),the temperature of the molten salt bath may be within a range from about400° C. to about 500° C., and the predetermined time period may bewithin a range from about 4 hours to 24 hours, for example in a rangefrom about 4 hours to 10 hours. Incorporation of larger ions into glasssubstrate 18 strengthens surfaces of the glass substrate by creating acompressive stress in a near-surface region. A corresponding tensilestress is induced within a central region of the glass substrate 18 tobalance the compressive stress.

By way of further example, sodium ions within glass substrate 18 may bereplaced by potassium ions from the molten salt bath, though otheralkali metal ions having a larger atomic radius, such as rubidium orcesium, may replace smaller alkali metal ions in the glass. According toparticular embodiments, smaller alkali metal ions in glass substrate 18may be replaced by Ag+ ions. Similarly, other alkali metal salts suchas, but not limited to, sulfates, halides, and the like may be used inthe ion exchange process. The replacement of smaller ions by larger ionsat a temperature below that at which the glass network can relaxproduces a distribution of ions across the surface of the glasssubstrate 18 that results in a stress profile. The larger volume of theincoming ion produces a compressive stress (CS) on the surface andtension (central tension, or CT) in the center region of the glasssubstrate 18. At optional step 114 the glass substrate after ionexchange is subjected to a final water rinse followed by drying.

It should be apparent from FIG. 1, for example, that the intensity ofthe scattered light as a function of propagation distance will bedependent at least on the number of “bounces” as the light propagatesgenerally along axis 38. Thus, light that is scattered nearer lightsource 16 will typically have a greater intensity than light that isscattered farther from light source 16. Moreover, intrinsic attenuationwithin the light guide will reduce the intensity of the propagatinglight. Accordingly, methods to control the scattering to account forthese optical losses can be applied. In one example, a deposition ofmicrometer-sized features after the etching can be used to control thelight scattering as a function of distance across a surface of the glasssubstrate. This deposition process can be implemented, for example, byink-jet printing, silk screening or other suitable deposition process.

As best shown in FIGS. 1, 2 and FIG. 6, etched first major surface 22can be provided with a pattern of deposited material comprising discretelayers 44 at step 116, hereinafter “dots” 44. The dots 44 suppressscattering at first major surface 22. As seen in FIG. 2, while locallyrandomized, dots 44 are arranged on a macro scale in a graduatedconfiguration such that the spatial density of dots, defined by apercent coverage of a unit surface area by the dot material, decreasesas one moves away from light source 16 along axis 46. For example, aspatial density of dots proximate first edge 26 can be at a percentcoverage of 75%, whereas a spatial density of dots proximate second edge28 can be at a percent coverage of 25%. It should be understood,however, that the spatial density of dots can be configured in differentpercentages based on the desired effect. In other examples the spatialdensity can vary in a range from about 90% to about 10%, in a range fromabout 60% to about 40%. The variation in spatial density of the dots canvary linearly or nonlinearly as necessary to match the light attenuationbetween the maximum spatial density and the minimum spatial density orthe desired optical effect. In comparison, the spatial density along aline parallel with first edge 26 can be substantially uniform, so thatno substantial gradient exists along axis 43. The plurality of dots 44may be configured to be non-overlapping. To avoid being visuallyapparent to an observer, the plurality of dots 44 may be transparent,and should have an index of refraction that is within ±10% of the indexof refraction of the underlying substrate. The closer the indexmatching, the more effective the dotsd will be at suppressingscattering. Accordingly, in some embodiments the index of refraction ofthe dots can be within ±5% of the index of refraction of the underlyingsubstrate. The spatial density of the dots can be varied depending uponhow many edges of the substrate are lighted by coupling light from asuitable light source. For example, using the example described supra,if the glass substrate is lighted from both first edge 26 and secondedge 28, the density of dots may be high at both first edge 26 andsecond edge 28, since the intensity of the light is greatest proximatethe light source, and decreasing from each edge as one moves toward themiddle of the glass substrate, midway between the two edges (assuming,for example, the light intensity at each edge is the same). A similarrationale applies if all four edges are lighted such that the lowestsuppression of scattering occurs within the central portion of the lightguide, yet a substantially uniform amount of light is extracted acrossthe surface of the glass substrate to produce substantially uniformillumination.

Each dot 44 may be formed by depositing a predetermined amount of asuitable material, e.g. a polymer resin, having an index of refractionapproximately equal to the index of refraction of the glass substrate towhich they are applied (e.g., within 10%), and at a prescribed positionon the etched surface of glass substrate 18. Any number of suitableresins may be employed. For example, one suitable resin is Accuglass®T-11 produced by the Honeywell Corporation. Accuglass® T-11 is a familyof heat-cured methylsiloxane polymer resins useful for planarization ofmedia, particular in the field of integrated circuits. Accuglass® T-11includes an index of refraction of 1.39 @ 633 nm. It should be noted,however, that dots 44 need not be a polymer resin. For example, dots 44could be a transparent conductive oxide material having a similar indexof refraction or any other material that includes an index of refractionclosely matching the index of refraction of the underlying substrate andis simultaneously suitably transparent. Deposition of dots 44 may beperformed by any suitable deposition technique compatible with thenature of the material deposited. For example, polymer resin can bedeposited by an ink jet printer, or by silk screen printing. Forexample, a FujiFilm® Dimatix™ 2831 ink jet printer with a 1 picoLiterprint head has been successfully employed for this purpose. In the caseof ink jet deposition, a desired dot configuration can be assembled as adigital bit map and provided to the printer. Each dot may comprise adiameter in a range from about 15 micrometers to about 30 micrometers,for example from about 20 micrometers to about 28 micrometers or fromabout 22 micrometers to about 26 micrometers. A maximum thickness t ofeach dot may be in a range from about 4 nm to about 10 nm, for example,from about 4 nm to about 8 nm. If the dots are deposited via an ink jetprinter, the volume of polymer resin can be in a range from about 1picoLiters (pL) to about 15 pL. Ink jet printing has so far been shownto be the most controllable and consistent method of depositing thedots. Moreover, the ability to easily program the deposition pattern,and if necessary make changes thereto, is particularly advantageous.After the deposition process is completed, the deposited resin may thenbe cured at an elevated temperature to harden the resin. If the resin isinstead curable by an ultraviolet light, the resin may be cured byexposure to the requisite wavelength of light. It should be noted thatthe volume of ink used during the deposition of the dots is controlledsuch that the ink does not protrude significantly above the surface ofthe substrate they have been deposited on.

As described supra, an area of the etched surface covered by a dot 44extracts less light through scattering than a non-covered area byenhancing total internal reflection (conversely, by suppressingscattering). Thus, the deposition of dots on a textured surface providesa means for controlling the brightness at specific locations on thetextured surface by controlling the percentage of surface area coveredby dots 44 as a function of position on the textured surface. Once thedots have been deposited, protective film 42 applied to the un-etchedsurface of the glass substrate may be removed if desired.

In still another aspect, production of substrates comprising a texturedsurface can be produced in a production-friendly manner that largelyeliminates messy wet chemical etching of individual substrates.

In a first step, a master mold can be produced in accordance with stepsdescribed previously, for example through wet chemical etching to obtaina substrate with a surface texture, the textured surface having an RMSsurface roughness equal to or less than 75 nm, for example in a rangefrom about 5 nm to about 75 nanometers and wherein a correlation lengthof the surface texture is equal to or less than 150 nanometers, forexample in a range from greater than zero to equal to or less than 150nanometers, or in a range from greater than zero to equal to or lessthan 100 nanometers.

In a second step, an uncured elastomer can be applied to the texturedsurface of the master mold to coat the textured surface. Once thetextured surface has been coated with the uncured elastomer, theelastomer can be cured in accordance with the curing method of theparticular elastomer. For example, if the elastomer is an ultravioletlight (UV) curable resin, the coated textured surface can be exposed toa suitable UV light from a UV light source to cure the elastomer. Inother embodiments, the uncured elastomer may be a thermoplastic materialthat is cured with exposure to heat. When curing is finished, the curedelastomer coating can be removed from the textured surface of the mastermold to produce an elastomer production mold in a third step.

To produce additional substrates including a textured surface comprisinga surface roughness and correlation length identical to the surfacetexture of the master mold, one or more substrates, for example glasssubstrates, can be coated with a suitable UV-curable resin on at leastone major surface of the one or more substrates. The UV-curable resinshould be selected such that upon curing, the UV-curable resin attainsan index of refraction that matches or nearly matches the index ofrefraction of the substrate (e.g. within ±10% of the index of refractionof the substrate). The production mold produced in the foregoing processmay then be pressed into the UV-curable resin, after which theUV-curable resin is cured by exposing the UV-curable resin to UV lightin accordance with the curing requirements of the particular resin. Oncethe UV-curable resin has been cured, the elastomer production mold canbe removed, leaving a substrate with at least one textured major surfacehaving an RMS surface roughness equal to or less than 75 nanometers, forexample in a range from about 5 nanometers to about 75 nanometers andwherein a correlation length of the surface texture is equal to or lessthan 150 nanometers, for example in a range from greater than zero toequal to or less than 150 nanometers, or in a range from greater thanzero to equal to or less than 100 nanometers.

In some embodiments the substrate may be a glass substrate. A thicknessof the glass substrate can be, for example, equal to or less than 3millimeters, for example, in a range from about 0.01 millimeters toabout 3 millimeters, in a range from about 0.01 millimeters to about 2millimeters, in a range from about 0.01 millimeters to about 1millimeters, in a range from about 0.01 millimeters to about 0.07millimeters. In some embodiments the glass substrate can be a flexibleglass substrate having a thickness in a range from about 0.01millimeters to about 0.3 millimeters, in a range from about 0.01millimeters to about 0.2 millimeters, or in a range from about 0.01millimeters to about 0.1 millimeters. The flexible glass substrate maybe a flexible glass ribbon. In some embodiments the flexible glassribbon can be in the form of a roll, wherein the UV-curable resin isapplied to at least one major surface of the flexible glass ribbon as orafter the flexible glass ribbon is dispensed from the roll.

FIG. 10 illustrates a process wherein at least one major surface of aflexible glass ribbon can be processed to produce at least one texturedmajor surface in a continuous manner. As shown in FIG. 10, a flexibleglass ribbon 120 is configured in a roll 122 at a pay-out station 124.The flexible glass ribbon comprises two longitudinal edges extendingalong a length of the flexible glass ribbon, the two longitudinal edgesdefining the lateral width of two major surfaces 126, 128 of theflexible glass ribbon. The flexible glass ribbon 120 is payed out fromroll 122 in a conveyance direction 130 along a predetermined conveyancepath and coating 132 of a suitable index-matched UV-curable resin isapplied to the least one major surface of the flexible glass ribbon, forexample major surface 126. The at least one major surface of theflexible glass ribbon may then be contacted with an embossing drum 134including a textured surface 136 and, simultaneous with the contacting,the index-matched resin is cured by exposing the resin to a UV lightfrom UV light sources 138. The embossing drum surface contacting theUV-curable resin can include, for example, a production moldmanufactured in accordance with the foregoing process, wherein theproduction mold is attached to a peripheral surface of embossing drum134. As the embossing drum rotates and the flexible glass ribboncontinues advancing along the predetermined conveyance path, theproduction mold ceases contact with the cured UV-curable resin and comesinto contact with uncured UV-curable resin in a continuous manner. Theupstream (leading) portion of the flexible glass ribbon, including apolymer surface layer comprising a textured surface, may then be takenup by receiving roll 140 at take up station 142.

In another embodiment, embossing drum 134 can have the appropriatetexture formed directly into a peripheral surface thereof. For example,a laser can be used to produce a texture directly on the outercircumferential peripheral surface of the embossing, such as by laseretching (engraving) or ablation. The laser may heat up the surface andsubsequently vaporize the surface of the embossing drum, or may ablatethe surface. In some examples a pulsed, short-duration laser beam can beused to produce pitting of the surface in a pattern programmed into thelaser controller, the pattern conforming to a predetermined surfaceroughness such as the surface roughness and correlation length describedherein.

Polymer films produced in a manner as described above, where a textureis applied to a surface thereof, can be used in a variety ofapplications. For example, in one such application, a transparent glasssubstrate including a polymer film comprising a texture can be used in aprojection viewing system. In such as system, an image screen isproduced comprising a glass substrate including textured surface aspreviously described. The textured surface may be formed on a firstsurface of the glass substrate itself, such as by etching, or thetextured surface may be formed in a polymer layer that was deposited ona surface of the first glass substrate. The glass screen may then bemounted, for example positioned in a frame or stand. A projector canthen project an image onto the glass screen. The light forming the imageis scattered by the texture present on the textured surface such thatthe image is visible on the glass screen. In some embodiments a secondglass substrate may be joined to the first glass substrate such that thetextured surface is sandwiched between the first glass substrate and thesecond glass substrate. The glass screen may include a surface roughnessequal to or less than 100 nanometers, for example an RMS surfaceroughness (R_(q)) in a range from about 5 nanometers to 75 nanometers,and a correlation length equal to or less than 150 nanometers, such asgreater than 0 nanometers but equal to or less than 150 nanometers. Insome examples the correlation length is greater than 0 nanometers butequal to or less than 100 nanometers.

FIG. 12 illustrates a projection system 200 comprising projection screen202 including a textured substrate 204. Projection screen 202 is shownas a free-standing structure supported in an upright posture by a base207. The textured substrate can be a glass substrate, a polymersubstrate or a combination of polymer and glass as discussed more fullybelow. An image 208 may be projected onto projection screen 202 fromprojecting device 210. A cover glass 218 may be positioned adjacenttextured substrate 204.

In one embodiment, shown in FIG. 13A, projection screen 202 comprisestextured glass substrate 204 including a surface texture as describedsupra. Textured glass substrate 204 may include, for example, a surface214 etched with an etchant as previously described to produce therequisite surface roughness (and correlation length if applicable) on atleast one surface of the glass substrate. An opposing surface, hereshown as surface 216, may or may not be etched to produce a desiredtexture. A cover substrate 218 may be bonded to the textured surface 214of glass substrate 204 if desired in order to protect textured surface206 from damage.

In another embodiment, shown in FIG. 13B, projection screen 202 maycomprise a glass layer, for example untextured glass substrate 205, anda polymer layer 212 disposed on the glass layer. Polymer layer 212includes a surface texture having the surface roughness characteristicsdescribed supra. For example, the surface texture can be produced via amicro or nano-replication technique as described above, or any othersuitable replication process. The surface texture can be applied to thepolymer layer after deposition of the polymer layer to the glasssubstrate, or the polymer layer may be added to the glass substrate, forexample as a pre-textured film. As shown in FIG. 12 and FIG. 13C, acover glass substrate 218 may be positioned over the polymer layer 212.Cover glass 218 may be configured with touch functionality.Alternatively, non-textured surface 216 may be provided with touchfunctionality.

In another aspect, a textured glass substrate configured to minimizeelectrostatic charging of the glass substrate, such as throughtriboelectrification that can occur during contact separation, isdisclosed.

Flat panel display glass used to build a display panel, and particularlythat portion of the display panel including thin film transistorsconsists of two sides, a functional side (“backplane”) upon which thinfilm transistors (TFTs) may be built (A-side) and a non-functionalB-side. During processing, the B-side glass contacts a variety ofmaterials (i.e. paper, metals, plastics, rubbers, ceramics, etc.) andcan accumulate an electrostatic charge through triboelectrification. Forexample, when the glass substrate is introduced into the production lineand an interleafing material, for example a laminated paper, is peeledfrom the glass substrate, the glass substrate can accumulate anelectrostatic charge. Moreover, during the manufacturing process forsemiconductor deposition, the glass substrate is commonly placed on achucking table, B-side in contact with the chucking table, where thedeposition is performed. The chucking table may, for example, restrainthe glass via one or more vacuum ports in the chucking table duringprocessing. When the glass substrate is removed from the chucking table,the B-side of the glass substrate can be electrostatically chargedthrough triboelectrification and/or contact electrification. Suchelectrostatic charge can cause many problems. For example, the glasssubstrate can be adhered to the chucking table by the electrostaticcharge and the glass substrate subsequently broken into pieces when anattempt is made to remove the glass substrate from the chucking table.Moreover, due to the electrostatic charge, particles and dust can beattracted to the glass surface and contaminate it. More importantly, therelease of electrostatic charge from the B-side to the A-side (so-calledelectrostatic discharge, ESD) can cause failure of the TFT gate and/orline damage on the A-side that reduces product yield.

Methods described herein may be used to minutely texture a glass surfaceby reducing he contact area in a manner that effectively reduces theintimacy of contact during triboelectrification and/or contactelectrification, resulting in reduced glass voltage or surface charge,without a noticeable reduction in the transparency of the glass, forexample with minimal haze. As previously described, an organic solventis introduced to an inorganic acid to produce rapid localizedprecipitation that forms crystal features on a glass substrate surface.These precipitates are the etching byproducts, normally fluorosilicatesalts, which mask the underlying glass surface and hinder etching inthese locations. Residual crystalline precipitates are dissolved awayduring a subsequent hot water wash or with an acid wash, leaving texturefeatures on the glass surface as a result of the etching. By adjustingthe organic solvent-to-etchant ratio or etch time or etchant temperaturea wide range of texture roughness can be obtained, from nanometer tomicrometer range.

Methods described herein can be applied to almost all of the traditionaletchants used in the glass industry. For example, the inorganic acidcould be hydrogen fluoride acid, buffered hydrogen fluoride acid (or themixture of ammonium fluoride and hydrogen fluoride), sodium fluoride andtheir mixtures with mineral acids including hydrogen chloride, sulfuricacid, phosphoric acid and nitric acid. The organic solvent must bemiscible with the etchant. Suitable organic solvents include withoutlimitation acetic acid, polyethylene glycol, isopropanol alcohol,ethanol, methanol, and others.

Accordingly, in some embodiments the processes described herein may bealtered as necessary. For example, if the intent is to reduceelectrostatic charging and not to produce a light guide, such aspects asviewing angle may not be important considerations. Additionally, changesin glass type and/or composition may further warrant adjustments to thechemistry involved in the etching process.

In one example process, a glass substrate is initially washed with a KOHsolution to remove organic contaminants and dust on the surface as apristine glass surface is needed to achieve uniformly distributedprotrusions (texture features) on the glass surface. Other washingsolutions may be substituted as needed. The presence of contaminants ordust on the surface can act as nucleation seeds. These nucleation seedscan induce crystallization around them, resulting in a non-uniformsurface texture.

Next, the glass substrate is soaked in a solution containing an organicsolvent in a range from about 10% by weight to about 90% by weight, forexample in a range from about 10% to about 80% by weight, in a rangefrom about 20% to about 70% by weight, in a range from about 20% byweight to about 30% by weight or in a range from about 40% by weight toabout 60% by weight. The etchant may further include an inorganic acidin a range from about 1% to about 50% by weight, for example in a rangefrom about 5% to about 40% by weight, or in a range from about 10% byweight to about 30% by weight. For example, in some embodiments asolution containing NH₄F in a range from about 1% to 20% by weight and5% to about 50% by weight acetic acid or polyethylene glycol can beselected. Depending on the desired texture roughness, the etching timecan range from about 30 seconds to a few minutes, for example in a rangefrom about 30 seconds to about 4 minutes, from about 30 seconds to about3 minutes or from about 30 seconds to about 2 minutes. The solution maybe static in some examples, while the solution can be stirred orotherwise agitated in other examples. For example, the etchant may berecirculated. Experiments have shown no discernible difference insurface texture based on etchant agitation within the etchantconcentrations and etch times described.

After etching is completed, the glass substrate can be soaked in 1MH₂SO₄ for up to 1 minute to remove crystal residues on the surface.However, the H₂SO₄ acid wash can be replaced by other mineral acids suchas HCl or HNO₃. Alternatively, or in addition, a hot water wash may beapplied. A low pH value (or high temperature) can increase thesolubility of the precipitated crystals. Consequently, a pH greater than7, for example greater than 8, is desired. After the wash the glasssubstrate may be rinsed with water to remove the acid residue, thendried.

The resulting average roughness found to be effective to reduce surfacevoltage (e.g. electrostatic charging), expressed as an average roughness(R_(a)) is typically in a range from about 0.4 nanometers to about 10nanometers. Typical haze values are equal to or less than 1% but may insome examples extend to as great as 6%. Accordingly, specific solutionrecipes as described above can be varied within the disclosed ranges asnecessary to attain a surface within the foregoing constraints.

Using the treatment methods described herein can result in a reductionof surface voltage exhibited by a glass substrate surface from about 20%to about 70%, for example in a range from about 23% to about 67%, overthe untreated substrate surface when tested via the disclosed peel test.In the context of reducing the danger from electrostatic discharge andthe potential damage to sensitive electronic components, such a changein acquired electrostatic charge is significant and useful.

However, further disclosure reveals a contact-area-based metric that canbe used to characterize the advantageous component of surface topographymost impactful to electrostatic charging behavior, and which describesregimes of ESD-advantaged surface topography not captured or predictedby simple parameters like average roughness (R_(a)).

It is known that surface topography can be quantitatively defined ordescribed using a variety of metrics over a variety of length scaleswith corresponding advantages and disadvantages to the different datacollection and quantification methods. Techniques like atomic-forcemicroscopy (AFM) or optical profilometry described elsewhere hereinprovide images with height (z) for each pixel measured in x and ydimensions. The size of the x-y pixel determines the ability of thetechnique to resolve small features, while the x-y field-of-view limitsthe ability to capture larger features (or changes in topography thatoccur over wider scales). An important note in this regard is that theessential character of surface texture can be well-described using somemetrics, but totally fail by other metrics. For example, if one had asurface texture covered by a regular array of 1 millimeter diameter dotsof 100 nanometer height, but collect images 50 nanometer×50 nanometer insize and quantify the pattern using only correlation length, one couldcompletely fail to capture the salient aspects of the topography. Thedegree to which this might be true depends on the behavior one wishes tounderstand.

With this mind, average roughness (R_(a)) or root mean square roughness(R_(q)) are some of the most commonly used parameters to describesurface topography, and yield information about the amplitude variationof heights on a surface (consistent with the traditional sense of“roughness”). In practice, however, this definition is only reallyappropriate when the topography being described has more-or-less“random” variations of heights on its surface. Ra and Rq fail to captureor suitably quantify the presence of “features” on a surface that onecan visibly see in an image, instead serving to average them out overthe field-of-view. It is thus not the most appropriate metric forquantifying the benefit of surface texture in terms of contactseparation when the surface contains topography other than randomroughness.

Other parameters like skewness or kurtosis also seek to quantify theasymmetry of the surface height profile about a mean surface level. Forexample, as described previously, negative skew indicates a predominanceof “valleys,” while positive skew is seen on surfaces with apredominance of “peaks.” Kurtosis measured the deviation from “random”amplitude variations, with 3 representing a normal/Gaussian distributionof heights. However, these single values also do not suitably describethe height distributions of nanometer-scale features on a surfaceconsidered important for contact separation, and tend to underestimatethe contribution of surface features, for example, when the features aresparsely distributed on a surface.

Accordingly, a metric is used that can define ESD-advantaged surfacetopographies in a way that captures their essential benefit throughenhanced contact separation. Hereafter we define this metric as afigure-of-merit (FOM) specific to contact separation, per the followingequation:

$\begin{matrix}{{FOMm} = {\frac{{CA} \cdot \delta^{m}}{N} \times {\sum\limits_{i = 1}^{N}\frac{1}{\Delta_{i}^{m}}}}} & (5)\end{matrix}$with:

$\begin{matrix}{\Delta_{i} = \left\{ \begin{matrix}{h_{t} - h_{i} + \delta} & {h_{i} \leq h_{t}} \\{h_{t} + \delta} & {h_{i} > h_{t}}\end{matrix} \right.} & (6)\end{matrix}$where the parameter h_(t) is a “threshold height”, effectively definingthe contribution of uppermost heights on a surface that comes intocontact with a second surface during a contact event. For example,h_(t)=0.95 corresponds to the top 5% of height values in an imagerepresents pixels that are “in contact” while h_(t)=0.50 considers thetop 50% of height values in an image represents pixels that are “incontact.”

An alternative way of thinking about this parameter is the “depth” towhich surface features elastically compress during a contact event, suchthat multiple pixels at the peak of a feature are rendered in contact,and thus not artificially restricted to the single topmost pixel at thetop of each peak.

The parameter δ is a “stand-off”, representing a minimum distancebetween the contacted area and an imaginary plane during a contactevent, Δ effectively describes for pixels not “in-contact” the extent ofseparation from the imaginary contact plane during a contact event, CAis the contact area, given by the number of pixels for whichh_(i)≥h_(t), m is the exponent of the denominator and weights theimportance of separation distance for pixels not “in-contact.” Inevaluations of FOM, m is selected to be either 2 (“FOM2”) or 6 (“FOM6”),respectively representing physics of Coulombic (˜r²) or Lennard-Jones(˜r⁶) interaction-length dependencies. The surface height data forindividual pixels is h_(i), where there are N total surface heightelements in the h_(i) data.

In plain language, the first term in equation (5) captures the area ofthe surface in “direct” contact with a secondary surface, in this casemathematically equivalent to an imaginary plane, during a contact event.The second term in the equation describes for regions of the surface notin “direct” contact how separated it is from the contacting surface. Inthe limit of improved ESD behavior, FOM approaches zero as (a) effectivesurface-to-surface contact-area is reduced (CA→0) and (b) those regionsnot in direct contact are as far away from the second surface aspossible (Δ→∞). Accordingly, in some examples a substrate, such as aglass substrate, includes at least one textured surface comprising anFOM in a range from 0 to 0.8, for example in a range from greater thanzero to about 0.78, in a range from about 0.01 to about 0.7, in a rangefrom about 0.05 to about 0.6, or in a range from about 0.01 to about 0.5for m in a range from 2 to 6 and wherein 0.1≤δ≤10 nm, for example0.1≤δ≤2 nm, and 0.5≤h_(t)≤0.999, for example 0.75≤h_(t)≤0.99.

It should be noted that the foregoing discussion in terms ofelectrostatic charging does not by itself address the optical qualitiesof the substrate should those optical properties be of interest. Thatis, FOM is directed toward minimizing electrostatic charging of asubstrate based on, for example, contact separation, and is equallyapplicable for transparent or opaque substrates (e.g. glass substrates).Accordingly, for instances where attention must be paid to atransparency of the substrate, the ESD characteristics must be balancedwith the optical qualities. To wit, a substrate having optimal surfaceproperties for electrostatic charging may still present less thanoptimal optical properties. Thus, obtaining a substrate comprising bothan acceptable FOM and acceptable transparency may require compromisebetween the two requirements, and while it has been shown that FOM isgenerally insensitive to roughness (either R_(a) or R_(q)), theseattributes may still have importance to transparency. In furtherexamples, a substrate comprising an FOM in a range from 0 to 0.8, forexample in a range from greater than zero to about 0.78, in a range fromabout 0.01 to about 0.7, in a range from about 0.05 to about 0.6, or ina range from about 0.01 to about 0.5 may also include an RMS roughnessR_(q) where 5 nanometers≤Rq≤75 nanometers, and wherein the texture ofthe textured major surface includes a correlation length T where 0nanometers<T≤150 nanometers.

EXAMPLES

The present disclosure will be further clarified by the followingexamples.

Example 1

Corning® Code 2320 glass samples were etched in solutions 1-6 fromTable 1. Brightness and haze were measured and the data are presentedbelow in Table 2. Code 2320 glass is a sodium alumino silicate glass.

TABLE 2 Decay exponent, Brightness @ 3 mm, Sample Solution mm⁻¹, b Watts% Haze 1 1 0.012 2.96E−07 0.791111 2 2 0.009 2.57E−07 0.346667 3 3 0.0123.03E−07 0.715556 4 4 0.013 4.09E−07 0.798889 5 5 0.012 4.75E−070.856667 6 6 0.021 6.39E−07 4.261111

As the data show, samples exposed to etchants 1-5 are similar in respectof the attenuation of light. A good metric to compare brightnessuniformity of the two conditions is b, the exponent in an exponentialfit (e.g., where the fit equation is of the form y=Ae^(bx)) of thebrightness data. The data of Table 2 describe a brightness decayexponent b ranging from about 0.009 mm⁻¹ to about 0.013 mm⁻¹ and withetchant 2 exhibiting the lowest brightness decay. The sample treatedwith etchant 6, although initially brighter than the others, exhibited asteeper decay curve with X=0.21 mm⁻¹. Samples made with solutions 1-5have less than 1% haze, indicating a very high transparency to light inthe visible wavelength range and minimal scatter. Etchant 6 had thehighest haze (4.26%), but was still useable in a transparent backlightelement.

The luminance in candelas per meter square as a function of viewingangle in degrees of the glass samples etched with solutions 1-6 above(curves 60, 62, 64, 66, 68, 70 respectively), for viewing angles from−80 to 80 degrees, was measured and is depicted in the plot of FIG. 14.This plot shows that luminance is most uniform for the glass sampleetched with etchant 2 (curve 62), and intermediate uniformity resultedfrom using etchants 1, 3, 4 and 5. Etchants 1 and 3 produced almostidentical results. Non-uniformity of luminance increased significantlyfor the sample treated using etchant 6.

Additionally, roughness parameters, including correlation lengths of thesamples treated with etchants 1, 2 and 6 through 9 were measured and arepresented below in Table 3, along with RMS roughness R_(q). The columns,from left to right, are the sample identification (7 through 17), theparticular etch conditions used (including glass temperature in degreescentigrade and etchant solution identification according to Table 1),roughness parameters as determined with an atomic force microscope (AFM)including R_(a) in nanometers, R_(q) in nanometers, skew (R_(sk)),kurtosis (R_(ku)), Gaussian correlation length (T) and a “viewing anglegoodness” (VAG) parameter. R_(a) is the arithmetic mean roughness value,R_(q) is the quadratic mean roughness value (RMS roughness), skew is ameasure of the asymmetry of a probability distribution. If skew<0, itcan be a surface with valleys and if R_(sk)>0 it can be a flat surfacewith peaks. Values numerically greater than 1.0 may indicate extremevalleys or peaks on the surface. Kurtosis is a measure of the randomnessof heights, and of the sharpness of a surface. A perfectly randomsurface has a value of 3; the farther the result is from 3, the lessrandom and more repetitive the surface is. Surfaces with spikes exhibithigher kurtosis values; bumpy surfaces exhibit lower kurtosis values.

Since most people watch a display (e.g. television) from a viewing anglein a range from about −30 degrees to about +30 degrees as measured froma normal to the display panel surface, the luminance (as measured incandela/meter²) should be high at those low angles. However, displaysare sometimes seen from higher angles, so high luminance from about −80degrees to about +80 degrees are also highly desired. Accordingly, adisplay with uniform luminance at angles of −80 to +80 degrees is saidto have high viewing angle uniformity. Displays lighted with a backlightelement acquire viewing angle uniformity primarily via the lightscattering properties of the backlight element.

When luminance is measured with equipment capable of measuring atvarious viewing angles, the results can be displayed as a spherical“radar” plot, or as an X/Y plot of viewing angle vs. luminance at a 90degree slice through the total hemispherical dataset, as shown in FIG.15 illustrating luminance as a function of viewing angle L(θ) for 12different backlights. Tools for acquiring viewing angle luminance may bea Radiant Zemax imaging sphere and an ELDIM EZ Contrast. Since comparingdesirability of viewing angle plots can be difficult, a metric calledviewing angle goodness (VAG) was developed. VAG is calculated by firstmultiplying the luminance curve, L(θ), by the cosine of the viewingangle θ to preferentially weight the light scattered at low viewingangles θ. The average luminance at a viewing angle θ from 0 to 30degrees is then divided by the average luminance at a viewing angle θfrom 30 to 80 degrees (see Equation 7 below).

$\begin{matrix}{{VAG} = \frac{{{Average}\left\lbrack {{L(\theta)}\cos\;\theta} \right\rbrack}|_{30}^{0}}{{{Average}\left\lbrack {{L(\theta)}\cos\;\theta} \right\rbrack}|_{80}^{30}}} & (7)\end{matrix}$

VAG is a quantitative metric that accounts for the “goodness” of theluminance as perceived by a viewer, for example a viewer of a displaylighted by a backlight element. The greater the VAG value, the greaterthe perceived goodness (acceptability) of the illumination.

The data from Table 3 show that generally the samples treated withetchants #1 and #2 (samples 8 to 11 and 14 in particular) have greaterVAG values (e.g. greater than 1.5), and Gaussian correlation lengths (T)substantially smaller than for the samples treated with etchants 6through 9. Generally, the smaller the correlation length the better,with correlation lengths less than 100 nm being considered the mostdesired. In particular, sample 11 processed with solution 2 had thehighest VAG value. A graph of the relationship between correlationlength and VAG is shown in FIG. 16.

TABLE 3 Etch Conditions Glass Temp, AFM Derived Roughness ParametersSample ° C. Solution Ra, nm Rq, nm Skew Kurtosis T VAG 7 −19 2 48.4 62.2−1.38 1.73 248.49 0.730499 8 37 2 25.6 33.4 −1.58 2.50 88.892 1.675984 960 2 16.1 21.8 −1.87 4.30 63.528 1.717703 10 22 1 23.6 31.6 −1.53 2.98115.65 1.740163 11 22 2 19.9 26.7 −1.37 2.75 97.24 1.865436 12 22 6 36.343.9 −0.57 −0.14 187.58 0.905445 13 4 2 19.4 26.0 −1.56 3.19 131.591.402849 14 22 2 (with 19.4 26.0 −1.56 3.19 98.318 1.558392 out water)15 22 7 19.2 25.7 −1.59 3.27 152.6 1.29123 16 22 9 19.3 25.9 −1.57 3.21185.22 0.925579 17 22 8 19.3 25.9 −1.57 3.22 251.53 0.644086

To better understand the surface morphology of the etched glass,porosity measurements were carried out on glass samples via argonsorption measurements using a Micromeritics® ASAP™ 2420 acceleratedsurface area and porosimetry system. The samples were all Corning code2320 glass (150 mm×150 mm×0.7 mm) manually dip-etched in acid solutionsand rinsed in deionized water. All samples were allowed to outgas for 24hours prior to measurement at 300° C. All pore size measurements werebased on the volume of argon adsorbed via the Barrett-Joyner-Halenda(BJH) model at a temperature of 87 K for pressures ranging from 4.72 to734.12 millimeters Hg absolute pressure. All pore size distributionfitting was conducted using a Rosin-Rammler model via the MicrosoftExcel Solver function to extract pore size distributions into theirrespective modes.

For the purpose of this study the foregoing method was employed todetermine the volume fraction of the two types of pores that appeared topresent in the overall pore size distribution: pores centered at analpha (a) value above or below 1,000 Angstroms (A) where alpharepresents the 63rd percentile of a Weibull distribution and correspondsgenerally to the peak value. The data is shown in Table 4 below. Foreach sample, the parsed pore volume fraction centered at 1,000 Angstromsis shown as the parameter X₁, while its counterpart population iscalculated as 1−X₁.

TABLE 4 Etch Glass Fraction Time, Temp., below “Bimodality” SampleSolution min ° C. VAG 1000 A [X₁] [X₁(1 − X₁)] 15 8 9 22 1.21 0.8870.100231 16 10 9 22 1.06 0.869 0.113839 17 9 7 22 1.01 0.87 0.1131 18 23.5 −19 0.776 0.113 0.100231 19 2 3.5 37 1.616 0.795 0.162975 20 2 3.560 1.599 0.292 0.206736

After calculation of these fractions, a “bi-modality” parameter BP wascreated by determining the product of these two volume fractions[X₁*(1−X₁)]. The bi-modality was then correlated with the VAG parameterusing a second order polynomial fitted to the data to determine anyempirical correlations. This fit, shown in FIG. 16, exhibits anempirical correlation with the VAG parameter. The data suggest aunitless bimodality parameter (BP) can adequately express a favorablesurface porosity for generating a scattering light guide with a high(≥1.0) VAG value. FIG. 17 suggests a BP value between about 0.16 and0.22 as being suitable for this purpose.

Example 2

In a series of experiments, 0.7 mm thick Corning code 2318 glass sampleshaving dimensions of 150 mm×150 mm were etched by immersing the samplesin a mixture of glacial acetic acid and aqueous ammonium fluoride.Corning code 2318 is an alkali alumino silicate glass. The samples wereetched in a bath with a component ratio of 15:6:4 of glacial aceticacid, water and ammonium fluoride, respectively. The etchant bathimmersion time was 3 minutes, after which the samples were rinsed inde-ionized water and air dried. The samples were lighted along one edgesurface using an LED strip and the brightness of the samples' output ata major surface thereof was measured starting from the edge nearest theLED strip across to the opposite edge.

The etched surfaces of the samples were then printed with a transparentresin in selected areas using a FujiFilm® Dimatix™ 2831 ink jet printerwith a 1 picoLiter print head, depositing droplets of resin, each resindroplet containing approximately 1 picoLiter of polymer resin. Thepolymer resin was Accuglass® T-11. The gradient patterns used to printthe polymer resin were designed using Adobe® Illustrator software andconverted to a bit map format compatible with the printer driver. Theprinted pattern was a (locally) random distribution of polymer resindots configured such that a coverage of dots per unit area proximate oneedge of each glass sample, i.e. the edge closed to the LED strip, wasgreater than the coverage of dots per unit area proximate the oppositeedge. The gradient thus produced was an approximately linear gradientfrom the first edge to the second edge. The printing resulted in dotsthat were about 15-47 microns in diameter. FIG. 18 is a photograph of asingle dot deposited on the roughed surface of the glass substrate, in atop-down view, and shows the dot to be of an approximately circularshape with a diameter of approximately 18 micrometers (μm). FIG. 19 is asecond photograph of another dot (having a more oval or egg-like shape).FIG. 20 is a plot of the profile of the dot of FIG. 19 showing the dotheight on the left (y) axis and the width on the bottom (x) axis alongline 1 of FIG. 19.

The plot in FIG. 21 compares sample brightness after etching (bothwithout dots), curve 56 to the brightness of the same sample etched andprinted with the T-11 resin, curve 58. The sample brightness wasdetermined by measuring an optical power adjacent the surface oppositethe etched surface as a function of distance from the light source. Thebrightness decay exponent b for the etched sample before printing was0.009 mm⁻¹ and the brightness decay exponent b for the etched and T-11printed samples was 0.005 mm⁻¹. Printing the T-11 resin on etchedsamples significantly reduced the brightness decay exponent, leading toimproved illumination uniformity across the etched and printed sample.

The roughness of the etched but not printed region measured an RMSroughness of 4 nanometers using a Zygo Newview™ model 7300 white lightinterferometer with a 100× lens and a 2× zoom. The dots resulting fromthe resin deposition increased the surface RMS roughness (R_(q)) toabout 8 nm.

Example 3

In one laboratory experiment glass substrate samples were cooled in afreezer to a temperature of about −19° C.) and a refrigerator to atemperature of about −4° C., heated to various temperatures in an oven,then immediately etched in a fresh solution #2 (as shown in Table 1),and rinsed. Glass temperature experiments were performed on two separatedates approximately 1 week apart, and strong positive log correlations(R²>0.95) were exhibited between glass temperature prior to etch and theVAG parameter, as seen in FIG. 22. However, this relationship was notthe same from one day to the next. It is believed that changes in therelative humidity, room temperature and dew point between the two datesmay have been responsible for differences in the glass temperature vs.VAG curves shown, and stabilizing the etching environments couldeliminate this variability.

Example 4

Samples of Corning code 2320 glass were etched with etchant solution #2from Table 1 for the times and at the temperatures shown below in Table5 below. Each sample was measured for haze and transmittance using a BYKHaze-Gard Plus instrument from the Paul N. Gardner Company, Inc. inaccordance with ASTM D 1003, ASTM D 1044. The Haze-Gard Plus is capableof directly determining total transmittance, haze and clarity. Theinstrument utilizes an Illuminant C light source representing averageday light with a correlated color temperature of 6774 K. The results areprovided below in Table 5, and indicate very low haze (for example, lessthan 2%) and high transmittance (greater than 94%, it being understoodthat the maximum transmittance is bounded by 100%), providing for ahighly transparent glass substrate after treatment with an etchant asdescribed herein.

TABLE 5 Etch Etch Etch time temp % Sample solution (minutes) ° C. % HazeTransmittance 21 #2 1 22 1.6 94 22 #2 1 22 0.86 94 23 #2 1 22 0.76 95 24#2 1 22 0.71 95

Example 5

To understand the performance of an etch wax, 2.5 grams polycaprolactone(Sigma a704105, MW 45k) were added to 50 milliliters acetic acid in a100 milliliter round bottom glass flask and stirred for 45 minutes in ahot water bath to dissolve the polycaprolactone and produce a 5%polycaprolactone mixture. The flask was then removed from the water bathand 0.9 milliliter deionized water was gradually added while stirring.The mixture was transferred to a Nalgene™ bottle, after which 2.1 gramsof ammonium fluoride was added and the mixture stirred for 1 hour. Uponcooling, the mixture formed a 5% polycaprolactone etch wax.

The etch wax was apply to a cleaned 150 millimeter×150 millimeter×2millimeter sample of Corning® Code 4318 glass manually using a heightadjustable Gardco Teflon coated drawdown bar, 10.16 cm wide, set at<0.0254 cm gap. Code 4318 is an alkali alumino silicate glass. Thesample was laminated with a polymer film to protect one major surface ofthe sample from the etchant. The etch wax coating was left on the glassfor 3.5 minutes, then rinsed in 3 water baths with agitation.

The glass sample was subsequently delaminated by removing the layer ofprotective film covering the non-etched major surface of the sample, andthe sample was dried.

Additionally, both 10% and 15% polycaprolactone etch waxes were alsomade using the same process. These required remixing using a planetarymixer after initial stir since after several days of sitting they becamequite viscous (would not flow). This indicates the solutions arethixotropic. After remixing, they were fine for draw down application.The samples for all three applications were then tested for VAG, percenthaze and average luminance as previously described. The data ispresented in Table 6 below.

TABLE 6 Average Luminance, Sample % polycaprolactone VAG % HazeCandelas/m² 25 5 1.74 0.67 Incomplete coating 26 5 1.74 0.67 477 27 101.73 1.14 652 28 15 1.87 0.41 Incomplete coating 29 15 1.79 0.47 422

It has been found that the etching processes described herein arecapable of producing a surface texture that significantly reduceselectrostatic charging, and subsequent discharge, of glass substrates,for example glass substrates used in the manufacture of other displaycomponents, including display panels.

Example 6

In one example exploring the effect of a textured glass surface onelectrostatic charge characteristics of a glass substrate, glasssubstrates were etched in a solution comprising ammonium fluoride (NH₄F)with a concentration of 1 weight percent and 5 weight percent, andacetic acid in a concentration in a range from about 10 weight percentand 90 weight percent. The results describe the impact of acidconcentration and etch time on glass roughness using a 2-factor fractaldesign developed using Minitab software. Table 7 lists the average glassroughness R_(a) of a sample of Corning® Lotus™ glass etched with theforegoing different acid concentrations and etch times of 30 seconds and120 seconds. Lotus™ represents a family of substantially alkali-freealumino silicate glasses having a high annealing point (for example,greater than 765° C.). A high annealing point can produce a low rate ofrelaxation, and hence comparatively small amounts of dimensional change,making the glass ideal for use as a backplane substrate inlow-temperature polysilicon processes. Glass surface roughness wasmeasured using a Zygo instrument over a scan area of 130 micrometers×180micrometers. The data show that a broad range of solution chemistry canbe used to obtain very low surface roughness.

TABLE 7 Acetic Acid NH₄F Etch Time Roughness Sample (vol. %) (wt. %)(sec) R_(a) (nm) 30 10 1 30 0.369 31 10 1 120 0.439 32 10 5 30 0.472 3310 5 120 0.361 34 90 1 30 0.717 35 90 1 120 9.287 36 90 5 30 7.270 37 905 120 6.662 Reference: 0.5M NH₄FHF, 90 sec., 0.610 40° C., Lotus ™

Example 7

A solution composing of 20 weight percent of ammonium fluoride (NH₄F)and 50 weight percent of acetic acid was used to etch Corning Lotus™display glass samples. The glass was dipped in the solution for thefollowing periods of time: 30 seconds, 60 seconds, 90 seconds, 120seconds and 180 seconds. Some glass characteristics, such as averageglass roughness (R_(a)), the maximum height of convex shapes on theglass surface (R_(z)), which is essentially a measure of the averagedistance between the highest peak and lowest valley in each samplinglength, glass haze and glass clarity obtained at different etch time aresummarized in Table 8 below. Table 8 shows that slightly changing theetch time using this solution dramatically increased both R_(a) andR_(z). As the etch time is extended to 120 seconds and 180 seconds,increased glass roughness values, 17.85 nanometers and 46.80 nanometers,respectively, were achieved. Visible haze was observed on the glasssurface at 180 seconds and the glass clarity was not acceptable for somedisplay applications. However, such a surface roughness, even with haze,can be useful for other potential applications such as an anti-glaresurface or anti-reflection surface, etc., and may also be used forbacklight application where transparency may not be required, such asfor standard opaque display back light elements. It can also be used forgeneral lighting purposes that do not require a high degree oftransparency.

TABLE 8 Etch Haze Voltage Sample time (s) R_(a) (nm) R_(z) (nm) (%)Clarity Reduction (%) 38 30  0.79  71.47 0.14 5 23 39 60  2.86 155.330.33 5 24 40 90  6.98 291.33 0.94 4 28 41 120 17.85 324.00 3.70 2 53 42180 46.80 386.67 16.30  1 67

The impact of etch time on surface topography was studied by atomicforce microscopy (AFM) with a scan size of 100 micrometer×100micrometer. It was found that glass roughness increased approximatelyexponentially as the etching time increased, and that the roughness wastunable within the nanometer to micrometer range through the methodsdisclosed herein. As the etching time is extended, the precipitantcrystals grow and more nucleation seeds are formed. As a result, denserand larger crystal patterns were observed on the AFM images depicted inFIGS. 23A-23C as the etching time was increased from 30 seconds to 60seconds and then 120 seconds. FIGS. 23A-23C illustrate the topography ofLotus™ glass surfaces etched in 50% acetic acid and 20% ammoniumfluoride for 30s, 60s and 120s over a mapping area of 100micrometers×100 micrometers. Concurrently, glass roughness was found tohave increased from 0.79 nanometers to 19.5 nanometers over the range ofetch times. In comparison to traditional etching (the reference solutionwithout using organic solvents shown in Table 7), the surface obtainedby the in situ mask method described herein is effective at reducingaccumulated charge on glass surface for at least two reasons. First, alarge range of surface roughness can be achieved whereas traditionaletching tends to plateau at slightly more than 1 nanometer, as shown inFIG. 24. FIG. 24 illustrates average roughness R_(a) as a function ofetch depth for samples of Corning® Eagle XG® glass and Corning® Lotus™glass and shows the plateauing of the depth of etch. R_(a) in FIG. 24was measured by AFM with a scan size of 2 micrometers×2 micrometers.Second, the method creates protrusions (peaks) on the glass surface,which enables a relatively smaller contact surface area compared to thedepressions obtained by traditional etching, which is absent an organiccomponent in the solution. It should be noted that for the currentapplication, the achieved effects are constrained by both anelectrostatic consideration and optical considerations. The glass mustexhibit reduced electrostatic charging while also being highly opticallytransparent with minimal haze. The small contact surface area ispreferred to more effectively reduce the accumulated charges on glasssubstrate.

Example 8

To better understand the effect of surface texture on electrostaticcharging, Corning Lotus™ glass samples (180 mm×230 mm×0.5 mm) wereetched in the foregoing NH₄F/acetic acid solution (50% acetic acid and20% ammonium fluoride at 25° C.) for varying amounts of time asindicated in Table 7 and surface voltage was measured with acommercially available lift tester manufactured by the HaradaCorporation using a grounded 304 stainless steel chuck plate. Referencesamples were etched in a 0.2M NaF and 1M H₃PO₄ etchant solution. A −36kPa vacuum was created against the glass samples through a single vacuumport in the chuck table and insulative Vespel pins (5 mm radius) wereused to lift the glass substrate samples from the chuck plate. The liftpin speed was 10 mm/sec. Three samples per etch time were sampled andrun in random order. Six lift cycles per sample were conducted withionization used between lift cycles to neutralize the samples. Valueswere reported at 80 mm pin height. The voltage probe was configured totrack with the glass substrate during lift pin movement. The glasssamples were cleaned using a wash with 4% SemiClean KG, and the samplesconditioned in a class 100 clean room for 1 hour prior to test executionat a relative humidity of about 13%. The chuck and pins were HEPAvacuumed and wiped down with DI cleanroom wipe 1 hour before testing. Asacrificial glass sample was used to contact clean the chuck and pins atthe start of testing, side B then side A. Individual measurements wereaveraged to produce the reported voltage change.

The percent reduction in voltage was calculated using the followingEquation (8):[(V _(o) −V)/V _(o)]×100,  (8)where V_(o) is the average voltage measured after contact separation ofthe glass sheet before treatment and V is the average voltage measuredafter contact separation of the glass sheet after treatment.

The glass voltage reduction in percent as a function of glass roughnessmeasured by AFM was plotted and is shown in FIG. 25. The glass that wasetched in the reference etchant (0.2M NaF/1M H₃PO₄ at 40° C. for 90seconds) is shown as the circle. The acetic acid/ammoniumfluoride-etched samples are represented by the square data points. Atriangle denotes a sample for which no surface treatment was conducted.The results indicate that the glass voltage was significantly reduced ina range from about 23% to about 67% as glass roughness varied from 0.79nanometers to 54 nanometers when using the in situ mask etching approachwhen compared to a traditional inorganic acid etchant.

Example 9

In manufacturing, glasses are traditionally wet etched using variousmeans such as dipping, spray, brush touched, etc. Accordingly, anotherexperiment was conducted to demonstrate that the disclosed method isapplicable to different agitation conditions. As shown in FIGS. 26A and26B, Lotus glass samples were etched in a solution comprising 11%ammonium bifluoride (NH₄FHF) and 25% polyethylene glycol 20%, PG, (FIG.26A) and a solution composed of 20% ammonium fluoride (NH₄F) and 50%acetic acid, AA. The roughness was measured by a Zygo instrument with ascan size of 180 micrometers×130 micrometers. Samples having visiblehaze on the glass substrate after treatment are circled. Glass sampleswere dipped in a static solution (symbol: diamond), while other sampleswere dipped in a stirred solution (symbol: square). The data indicate nosignificant difference in respect of the resulting glass roughness.

For further comparison, the etchant was changed to ammonium bifluoride,ABF, (NH₄FHF) and polyethylene glycol (PG) as the organic solventcomponent, which was again used to etch Corning Lotus glass (FIG. 26B).Similarly, the experiment was conducted in both static and stirringsolutions. As expected, the same magnitude of glass roughness isachieved in both conditions. This example implies that the method isapplicable to either static or dynamic etching condition and isapplicable to any combination of fluorinated acid or salt and an organicsolvent (i.e., acetic acid, polyethylene glycol).

Example 10

As previously described, for display applications the electrostaticcharge-enhanced glass must perform satisfactorily both electrostaticallyand optically. Generally, roughening a glass surface may facilitate areduction of accumulated electrostatic charge on the glass substrate,but at the same time it may cause the optical problems due to thescattering of light on a rough surface. For display applications, theglass should typically be transparent, although electrostatic charge maybe considered even for opaque substrates. The glass clarity wastherefore evaluated by visual inspection by lighting the glass samplesfrom an edge surface thereof, and a clarity value assigned. While thisapproach is admittedly subjective, it was adequate to evaluate clarityon a relative basis. Glass substrate samples were etched in a firstsolution of 20% by weight NH₄F and 50% by weight acetic acid. Additionalglass samples were etched in a second solution of 11% by weight NH₄FHFand 25% by weight polyethylene glycol. It was observed that for a 90second exposure to the etchant the obtained average glass roughness(R_(a)) was 6.98 and haze was 0.94% (see Table 8), and the glass claritywas degraded. Haze was measured for each sample and plotted as afunction of glass roughness and is shown in FIGS. 27A and 27B. FIG. 27Adepicts haze as a function of R_(a) obtained through in situ masketching approach with the first solution of 20% by weight NH₄F and 50%by weight acetic acid. Glass surface roughness was measured by AFM witha scan size of 100 micrometers×100 micrometers. FIG. 27B plots haze as afunction of R_(a) for glass etched in the second solution of 11% byweight NH₄FHF and 25% by weight polyethylene glycol. Glass surfaceroughness was measured by a Zygo instrument with a scan size of 180micrometers×130 micrometers. The dashed line represents a haze valueequals to 1%, considered to be an optimal maximum haze for someapplications. With the exception of several outlier examples, the datashow that glass surface haze can be controlled within an acceptablerange, e.g. equal to or less than about 6%. The results further suggestthat to maintain optimal clarity while at the same time facilitating areduction in electrostatic charging, the glass roughness should becontrolled in a range from about 0.4 nanometer to about 10 nanometer,with haze equal to or less than about 1%.

The glass surface composition for the samples was then analyzed byscanning electron microscopy-electron dispersive X-ray spectrometry(SEM-EDX). The composition of the surface protrusions (peaks) was foundto be identical to the glass substrate composition. There was no etchingby-product residue, (NH₄)₂SiF₆, found on the glass surface, meaning theacid rinse after the etching step efficiently removed the in situ maskmaterial from the glass surface. This result supports a conclusion thatthe addition of an organic solvent to an etchant solution facilitatesthe nucleation of crystals, the etching byproduct, on the glass. Thesecrystals mask the underlying glass surface and hinder etching in theselocations. Residual crystalline precipitates can then be dissolved awayduring a subsequent acid wash or hot water wash, leaving texturedfeatures on the glass surface.

Example 11

In another experiment, the performance of the FOM metric and itscorrelation to ESD response was evaluated. Glass substrate samples wereetched with a variety of etchants. All samples were 180 millimeters×230millimeters×0.5 millimeters of Corning® Lotus™ glass. The glasssubstrates were then subjected to a lift test and surface voltage wasmeasured.

A first etchant comprised 1.5M HCl and 1.5 M HF produced in a 7 literbatch by combining 862 milliliters HCl, 381 milliliters HF and 5757milliliters deionized H₂0. The samples were installed together in acarriage and bathed in the first etchant at a temperature of 30° C. for60 seconds, then rinsed in deionized water at room temperature for 30seconds.

A second etchant comprised 1M H₃PO₄ and 0.35M NaF produced in a 7 literbatch by combining 479 ml H₃PO₄, 103 grams NaF and 6521 ml deionizedH₂0. The samples were installed together in a carriage and bathed in thesecond etchant at a temperature of 40° C. for 81 seconds, then rinsed indeionized water at room temperature for 30 seconds.

A third etchant comprised 20% by weight NH₄F and 50% by volume aceticacid produced in a 1 liter batch by combining 200 grams NH₄F, 500 mlacetic acid and 300 milliliters deionized H₂0. The samples wereinstalled together in a carriage and bathed in the third etchant at roomtemperature for 40 seconds, then rinsed in 1 liter of an acid solutioncomprising produced by combining 56.1 milliliters H₂SO₄ and 943.9milliliters deionized water at room temperature for 30 seconds. Thisacid rinse was followed by further rinsing in deionized water.

A fourth etchant comprised 20% by weight NH₄F and 50% by volume aceticacid produced in a 1 liter batch by combining 200 grams NH₄F, 500 mlacetic acid and 300 milliliters deionized H₂0. The samples wereinstalled together in a carriage and bathed in the fourth etchant atroom temperature for 80 seconds, then rinsed in 1 liter of an acidsolution comprising produced by combining 56.1 milliliters H₂SO₄ and943.9 milliliters deionized water at room temperature for 30 seconds.This acid rinse was followed by further rinsing in deionized water.

A fifth etchant comprised 4.1% by weight NH₄F and 94.1% by weight aceticacid produced in a 1000 gram batch by combining 41 grams NH₄F, 896milliliters acetic acid and 18 milliliters deionized H₂0. The sampleswere installed together in a carriage and bathed in the fifth etchant atroom temperature for 210 seconds, then rinsed in deionized water at roomtemperature for 30 seconds.

Shown in FIG. 28A through 28F are AFM images at feature-appropriateX/Y/Z scales the electrostatic charging results (FIG. 29) for thedifferent conditions highlighting the progression of reducing theelectrostatic-induced voltage as a function of surface preparation,include topographies that represent “random” roughness, as well as somecontaining obvious topographic features.

After etching the glass substrates were subjected to a lift test with acommercially available lift tester manufactured by the HaradaCorporation using a grounded 304 stainless steel chuck plate. Surfacevoltage was measured during the lift testing. A −36 kPa vacuum wascreated against the glass samples through a single vacuum port in thechuck table and insulative Vespel pins (5 mm radius) were used to liftthe glass substrate samples from the chuck plate. The lift pin speed was10 mm/sec. Four samples per etch time were sampled and run in randomorder. Six lift cycles per sample were conducted with ionization usedbetween lift cycles to neutralize the samples. Values were reported atan 80 mm pin height. The voltage probe was configured to track with theglass substrate during lift pin movement. The glass samples were cleanedusing a wash with 4% SemiClean KG, and the samples conditioned in aclass 100 clean room for 1 hour prior to test execution at a relativehumidity of about 12%-13%. The chuck and pins were HEPA vacuumed andwiped down with DI cleanroom wipe 1 hour before testing. A sacrificialglass sample was used to contact clean the chuck and pins at start oftesting, side B then side A. Individual measurements were averaged toproduce the reported peak voltage change.

Peak surface voltage in kilovolts for the samples is depicted in FIG.29, shown with 95% confidence limits. The samples are referenced alongthe bottom (x-axis) as C0 through C5, where C0 refers to a controlsample that did not undergo etching, and where C1 through C5 refer tothe first through fifth etchants. It should be apparent from FIG. 29that all five etchants had a noticeable effect on electrostaticcharging. However, it should also be noted that the samples etched withan inorganic acid and an organic solvent (ammonium fluoride and aceticacid, respectively), produced significantly better results that etchants1 through 3.

In addition to voltage testing, 50 millimeter×50 millimeter samples ofthe etched glass were tested for haze and transmittance using aHaze-Gard Plus instrument, and the data is presented in Table 9 below.In this instance clarity was a measured quantity obtained directly fromthe Haze-Gard instrument and not a subjective grading as previouslyperformed. The samples were also measured for Ra, Rq correlation lengthT. The data were also used to calculate FOM for m=2.

TABLE 9 R_(a) R_(q) Sample (nm) (nm) T (nm) FOM(2) Haze TransmittanceClarity C0 0.28 0.34 6.20 0.77 0.12 94.3 100 C1 0.68 0.85 18.77 0.610.10 94.3 100 C2 0.57 0.72 14.97 0.64 0.07 94.3 100 C3 1.02 3.93 177.760.70 0.12 94.3 100 C4 7.49 17.07 357.34 0.66 1.08 94.3 99.8 C5 42.454.92 283.40 0.22 17.3 94.3 99.8

As is apparent from Table 9, all the sample sets exhibited good hazeperformance except for sample set C5. Good haze performance from thecontrol samples, C0, was expected, since the surfaces were pristine anduntreated. Sample sets C1 through C4 exhibit some improvement inelectrostatic charging performance, as represented by a reduction inpeak voltage when compared to the control sample set C0. However, sampleset C5, while showing the greatest improvement in peak voltagereduction, also showed a high haze value. Visual examination of the C5samples revealed noticeable haze. The high haze value for C5 compared tothe results attributable for earlier surface treatment with comparablesolutions used for light guide substrates can be attributed to thedifferences in glass compositions, further reinforcing the need to adaptthe chemistry to account for differences in glass composition.

For these data depicted in FIGS. 28A-F and 29, a series of plots areshown in FIGS. 30A, 30B, 31A, 31B demonstrating the correlation betweenthe electrostatically-induced voltage and FOM for specific values of m,h_(t) (“thr”), and standoff 8 (“offset”). Correlation coefficients (R²)are greater than 0.75 indicating substantial correlative value to themetric.

To optimize the correlative value of a given topographical metric, it ishelpful to explore the impact of various mathematical parameters goinginto its calculation, such as m, h_(t), and δ. In the contour plotsdepicted in FIGS. 31A, 31B, the variation in the correlation coefficientas a function of h_(t) and δ for two values of m (FOM2, FOM6) ishighlighted. In the plots, the x-axis “1−threshold” equates to(1−h_(t)), and the y-axis “offset” equates to δ. For these particulartopographies, the shaded regions represent a selection of h_(t) and δvalues that tend to maximize the strength of the correlation. The blackdots indicate examples of particularly high high correlation points.

The foregoing analysis shows that choosing the right h_(t) is a strongerfactor than δ in having FOM correlate and predict an electrostaticcharging response. Typical values of h_(t) might range from 0.7 to 0.9,while δ ranges from 0.5 to 2.0, and m=2 provides a higher R² value thanm=6, though the fit of FOM2 to electrostatic charge data is somewhateccentric. As with any topographical metric, the selection of“best-fitting” parameters like threshold heights may be influenced byspecific topographical feature dimensions, such that appropriate valuescan be chosen for a given surface texture with some judgment. Thispractice can even be performed when calculating R_(a) on a surfacecontaining features, since an underlying parameter in the calculation isthe selection of a suitable image size (e.g., for R_(a) to bemeaningful, one needs an image suitable to capture the features ofinterest. That is, if one is trying to calculate the roughness of arange of mountains, it is inappropriate to use data zoomed in on one ofthe mountainsides.

The unique space wherein FOM and R_(a) (R_(q)) diverge can be readilydescribed using the idealized topographies shown in FIGS. 32A, 32B, 33A,33B, 34A and 34B. FIGS. 32A-34B illustrate alternately pairs of surfacefeatures in a surface plane (x-y plane), and a corresponding slice inthe vertical (height) plane along the x-axis (height profile on thez-axis).

For a given R_(a) being held constant, FOM can vary almost independentlyfrom the limit of “pinholes” to “spikes” simply by varying the lateralfeature size. Likewise, at a given FOM (i.e. contact-area), R_(a) canvary almost independently by simply increasing the depth of the pinholes(or the height of the spikes). When short, narrow spikes are mixed withrandom roughness, it is also likely to create a scenario where thecontribution of the spikes will hardly be detected in the calculation ofR_(a). However, in all situations a surface with a multitude of sharpspikes providing contact separation will anticipate a much betterelectrostatic charging response than a surface featuring pinholes orrandom roughness alone.

This is illustrates graphically in FIGS. 35 and 36. FIG. 35 depicts FOM(m=2) as the “duty cycle” of the idealistic features (waveforms)depicted in FIGS. 32b , 33B and 34B vary (essentially, as the peaksbroaden). The top curve shows the progression of a high FOM (m=2) as itvaries from 1 toward a midpoint 0.5, whereas the bottom curve shows theprogression of a low FOM (m=2) as it varies from zero toward a midpoint0.5. The plot shows that for a constant peak-to-valley height(roughness), as the peaks broaden (and hence the contact area betweenthe substrate and a contacting surface increases), the FOM varies. Onthe other hand, FIG. 36 shows that for a broad range of roughness, foreach the conditions of FIG. 35 (varying duty cycle), the FOM (m=2), isconstant, illustrating that FOM is independent of roughness.

As a whole, the variety of FOM and R_(a) (R_(q)) responses are depictedschematically in FIG. 37, showing a regime where FOM & R_(a) (R_(q))mostly correlate well, but also highlighting the topographies whereR_(a) would fail to correlate with electrostatic charging response.

For the embodiments described above, both R_(a) (R_(q)) actually stillcorrelate with electrostatic charging response, and cross-correlate withFOM. This happens to be true for the surface topographies on theseparticular embodiments. However, it is not true as a rule, and there areclearly topographies for which FOM and R_(a) (R_(q)) will diverge. Inthese instances, FOM will serve as a better correlative descriptor forreduced contact area.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to disclosed embodimentswithout departing from the spirit and scope of the disclosure. Thus itis intended that the present disclosure cover the modifications andvariations of these embodiments provided they come within the scope ofthe appended claims and their equivalents.

What is claimed is:
 1. A substrate comprising at least one texturedmajor surface comprising a figure of merit FOM in a range from greaterthan zero to about 0.78, wherein FOM is calculated as${FOM} = {\frac{{CA} \cdot \delta^{m}}{N} \times {\sum\limits_{i = 1}^{N}{\frac{1}{\Delta_{i}^{m}}\mspace{14mu}{and}}}}$$\Delta_{i} = \left\{ {\begin{matrix}{h_{t} - h_{i} + \delta} & {h_{i} \leq h_{t}} \\{h_{t} + \delta} & {h_{i} > h_{t}}\end{matrix},} \right.$ where δ represents a minimum distance between acontacted area of the textured major surface with a contact plane forwhich 0.1≤δ≤10 nm, Δ represents the extent of separation from thecontact plane, CA is the contact area given by the number of pixels forwhich h_(i)≥h_(t), h_(i) represents the surface height data for Nsurface height elements, h_(t) represents a threshold height definingthe contribution of uppermost heights on the textured surface that comeinto contact with the contact plane for which 0.5≤h_(t)≤0.999, and mweights the importance of separation distance for pixels not contactingthe contact plane for which m is a whole number in a range from 2 to 6.2. The substrate according to claim 1, wherein the substrate comprises atransmittance equal to or greater than 94%.
 3. The substrate accordingto claim 1, wherein the substrate comprises a haze equal to or less than6%.
 4. The substrate according to claim 3, wherein the haze is equal toor less than 1%.
 5. The substrate according to claim 1, wherein anaverage roughness R_(a) of the textured surface is 0.4≤R_(a)≤10 nm. 6.The substrate according to claim 1, wherein a correlation length T ofthe surface texture is equal to or less than 150 nm.
 7. The substrateaccording to claim 6, wherein an RMS roughness R_(q) of the texturedsurface is 5 nm≤R_(q)≤75 nm.
 8. The substrate according to claim 1,wherein the substrate is a glass substrate.
 9. The substrate accordingto claim 1, wherein m is
 2. 10. The substrate according to claim 1,wherein m is
 6. 11. The substrate according to claim 1, wherein 0.1≤δ≤2nm.
 12. The substrate according to claim 11, wherein 0.5≤δ≤2 nm.
 13. Thesubstrate according to claim 1, wherein 0.75≤h_(t)≤0.99.
 14. Thesubstrate according to claim 1, wherein 0.7≤h_(t)≤0.9.
 15. The substrateaccording to claim 1, wherein the FOM is in a range from 0.22 to 0.77.16. The substrate according to claim 15, wherein the FOM is in a rangefrom 0.61 to 0.77.
 17. The substrate according to claim 1, wherein thesubstrate comprises a transmittance in a range from greater than 94% to100%.